Sildenafil, sold as the brand name Viagra among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension. Its effectiveness for treating sexual dysfunction in women has not been demonstrated.
|Trade names||Viagra, Revatio, others|
|By mouth (tablets), IV|
|Metabolism||Liver: CYP3A4 (major route), CYP2C9 (minor route)|
|Metabolites||N-desmethylsildenafil (~50% potency for PDE5)|
|Biological half-life||3–4 hours|
|Excretion||Feces (~80%), urine (~13%)|
|Chemical and physical data|
|Molar mass||474.5764 g/mol|
|3D model (JSmol)|
|(what is this?)|
Common side effects include headaches and heartburn, as well as flushed skin. Caution is advised in those who have cardiovascular disease. Rare but serious side effects include prolonged erections, which can lead to damage to the penis, and sudden-onset hearing loss. Sildenafil should not be taken by people who take nitrates such as nitroglycerin (glycerin trinitrate), as this may result in a severe and potentially fatal drop in blood pressure.
Pfizer scientists Andrew Bell, David Brown, and Nicholas Terrett originally discovered sildenafil as a treatment for various cardiovascular disorders. Since becoming available in 1998, sildenafil has been a common treatment for erectile dysfunction; its primary competitors are tadalafil (trade name Cialis) and vardenafil (Levitra).
The primary indication of sildenafil is treatment of erectile dysfunction (inability to sustain a satisfactory erection to complete intercourse). Its use is now one of the standard treatments for erectile dysfunction, including for men with diabetes mellitus.
Antidepressant-associated sexual dysfunction
Sildenafil and other PDE5 inhibitors are used off-label to alleviate vasospasm and treat severe ischemia and ulcers in fingers and toes for people with secondary Raynaud's phenomenon; these drugs have moderate efficacy for reducing the frequency and duration of vasospastic episodes. As of 2016,[update] their role more generally in Raynaud's was not clear.
In clinical trials, the most common adverse effects of sildenafil use included headache, flushing, indigestion, nasal congestion, and impaired vision, including photophobia and blurred vision. Some sildenafil users have complained of seeing everything tinted blue (cyanopsia). Some complained of blurriness and loss of peripheral vision. In July 2005, the FDA found that sildenafil could lead to vision impairment in rare cases and a number of studies have linked sildenafil use with non-arteritic anterior ischemic optic neuropathy.
Rare but serious adverse effects found through postmarketing surveillance include prolonged erections, severe low blood pressure, myocardial infarction (heart attack), ventricular arrhythmias, stroke, increased intraocular pressure, and sudden hearing loss. In October 2007, the FDA announced that the labeling for all PDE5 inhibitors, including sildenafil, required a more prominent warning of the potential risk of sudden hearing loss.
Care should be exercised by people who are also taking protease inhibitors for the treatment of HIV infection. Protease inhibitors inhibit the metabolism of sildenafil, effectively multiplying the plasma levels of sildenafil, increasing the incidence and severity of side effects. Those using protease inhibitors are recommended to limit their use of sildenafil to no more than one 25 mg dose every 48 hours. Other drugs that interfere with the metabolism of sildenafil include erythromycin and cimetidine, both of which can also lead to prolonged plasma half-life levels.
The use of sildenafil and an α1 blocker (typically prescribed for hypertension or for urologic conditions, such as benign prostatic hypertrophy) at the same time may lead to low blood pressure, but this effect does not occur if they are taken at least 4 hours apart.
- Concomitant use of nitric oxide donors, organic nitrites and nitrates, such as:
- Concomitant use of soluble guanylyl cyclase stimulators, such as riociguat
- Known hypersensitivity to sildenafil
Sildenafil's popularity with young adults has increased over the years. Sildenafil's trade name, Viagra, is widely recognized in popular culture, and the drug's association with treating erectile dysfunction has led to its recreational use. The reasons behind such use include the belief that the drug increases libido, improves sexual performance, or permanently increases penis size. Studies on the effects of viagra when used recreationally are limited, but suggest it has little effect when used by those not suffering from erectile dysfunction. In one study, a 25-mg dose was shown to cause no significant change in erectile quality, but did reduce the postejaculatory refractory time. This study also noted a significant placebo effect in the control group.
Unprescribed recreational use of sildenafil and other PDE5 inhibitors is noted as particularly high among users of illegal drugs. Sildenafil is sometimes used to counteract the effects of other substances, often illicit. Some users mix it with methylenedioxymethamphetamine (MDMA, ecstasy), other stimulants, or opiates in an attempt to compensate for the common side effect of erectile dysfunction, a combination known as "sextasy", "rockin' and rollin'" or "trail mix". Mixing with amyl nitrite is particularly dangerous and potentially fatal.
Jet lag research
The 2007 Ig Nobel Prize in Aviation went to Patricia V. Agostino, Santiago A. Plano, and Diego A. Golombek of Universidad Nacional de Quilmes, Argentina, for their discovery that sildenafil helps treat jet lag recovery in hamsters.
Acetildenafil and other synthetic structural analogs of sildenafil which are PDE5 inhibitors have been found as adulterants in a number of "herbal" aphrodisiac products sold over-the-counter. These analogs have not undergone any of the rigorous testing that drugs like sildenafil have passed, and thus have unknown side-effect profiles. Some attempts have been made to ban these drugs, but progress has been slow so far, as, even in those jurisdictions that have laws targeting designer drugs, the laws are drafted to ban analogs of illegal drugs of abuse, rather than analogs of prescription medicines. However, at least one court case has resulted in a product being taken off the market.
The US FDA has banned numerous products claiming to be Eurycoma longifolia that, in fact, contain only analogs of sildenafil. Sellers of such fake herbals typically respond by just changing the names of their products.
Detection in biological fluids
Sildenafil and/or N-desmethylsildenafil, its major active metabolite, may be quantified in plasma, serum, or whole blood to assess pharmacokinetic status in those receiving the drug therapeutically, to confirm the diagnosis in potential poisoning victims, or to assist in the forensic investigation in a case of fatal overdose.
Mechanism of action
Sildenafil protects cyclic guanosine monophosphate (cGMP) from degradation by cGMP-specific phosphodiesterase type 5 (PDE5) in the corpus cavernosum. Nitric oxide (NO) in the corpus cavernosum of the penis binds to guanylate cyclase receptors, which results in increased levels of cGMP, leading to smooth muscle relaxation (vasodilation) of the intimal cushions of the helicine arteries. This smooth muscle relaxation leads to vasodilation and increased inflow of blood into the spongy tissue of the penis, causing an erection. Robert F. Furchgott, Ferid Murad, and Louis Ignarro won the Nobel Prize in Physiology or Medicine in 1998 for their independent study of the metabolic pathway of nitric oxide in smooth muscle vasodilation.
Sildenafil is a potent and selective inhibitor of cGMP-specific phosphodiesterase type 5 (PDE5), which is responsible for degradation of cGMP in the corpus cavernosum. The molecular structure of sildenafil is similar to that of cGMP and acts as a competitive binding agent of PDE5 in the corpus cavernosum, resulting in more cGMP and better erections. Without sexual stimulation, and therefore lack of activation of the NO/cGMP system, sildenafil should not cause an erection. Other drugs that operate by the same mechanism include tadalafil (Cialis) and vardenafil (Levitra).
Sildenafil is broken down in the liver by hepatic metabolism using cytochrome p450 enzymes, mainly CYP450 3A4(major route), but also by CYP2C9 (minor route) hepatic isoenzymes. The major product of metabolisation by these enzymes is N-desmethylated sildenafil, which is metabolised further. This metabolite also has an affinity for the PDE receptors, about 40% of that of sildenafil. Thus, the metabolite is responsible for about 20% of sildenafil's action. Sildenafil is excreted as metabolites predominantly in the feces (about 80% of administered oral dose) and to a lesser extent in the urine (around 13% of the administered oral dose). If taken with a high-fat meal, absorption is reduced; the time taken to reach the maximum plasma concentration increases by around one hour, and the maximum concentration itself is decreased by nearly one-third.
Route of administration
- When taken by mouth sildenafil for erectile dysfunction results in an average time to onset of erections of 27 minutes (ranging from 12 to 70 minutes).
- Under the tongue use of sildenafil for erectile dysfunction results in an average onset of action of 15 minutes and lasting for an average of 40 minutes.
There are also mouth spray preparations of sildenafil for faster onset of action.
The preparation steps for synthesis of sildenafil are:
- Methylation of 3-propylpyrazole-5-carboxylic acid ethyl ester with hot dimethyl sulfate
- Hydrolysis with aqueous sodium hydroxide (NaOH) to free acid
- Nitration with oleum/fuming nitric acid
- Carboxamide formation with refluxing thionyl chloride/NH4OH
- Reduction of nitro group to amino group
- Acylation with 2-ethoxybenzoyl chloride
- Sulfonation to the chlorosulfonyl derivative
- Condensation with 1-methylpiperazine.
Sildenafil (compound UK-92,480) was synthesized by a group of pharmaceutical chemists working at Pfizer's Sandwich, Kent, research facility in England. It was initially studied for use in hypertension (high blood pressure) and angina pectoris (a symptom of ischaemic heart disease). The first clinical trials were conducted in Morriston Hospital in Swansea. Phase I clinical trials under the direction of Ian Osterloh suggested the drug had little effect on angina, but it could induce marked penile erections. Pfizer therefore decided to market it for erectile dysfunction, rather than for angina; this decision became an often-cited example of drug repositioning. The drug was patented in 1996, approved for use in erectile dysfunction by the FDA on 27 March 1998, becoming the first oral treatment approved to treat erectile dysfunction in the United States, and offered for sale in the United States later that year. It soon became a great success: annual sales of Viagra peaked in 2008 at US$1.934 billion.
The British press portrayed Peter Dunn and Albert Wood as the inventors of the drug, but only Andrew Bell, David Brown, and Nicholas Terrett are listed on the original composition of matter patent.
Society and culture
Marketing and sales
In the US even though sildenafil is available only by prescription from a doctor, it was advertised directly to consumers on TV (famously being endorsed by former United States Senator Bob Dole and football star Pelé). Numerous sites on the Internet offer Viagra for sale after an "online consultation", often a simple web questionnaire. The Viagra name has become so well known, many fake aphrodisiacs now call themselves "herbal viagra" or are presented as blue tablets imitating the shape and colour of Pfizer's product. Viagra is also informally known as "vitamin V", "the blue pill", or "blue diamond", as well as various other nicknames.
In 2000, Viagra sales accounted for 92% of the global market for prescribed erectile dysfunction pills. By 2007, Viagra's global share had plunged to about 50% due to several factors, including the entry of Cialis and Levitra, along with several counterfeits and clones, and reports of vision loss in people taking PDE5 inhibitors.
In February 2007, it was announced that Boots, the UK pharmacy chain, would try over-the-counter sales of Viagra in stores in Manchester, England. Men between the ages of 30 and 65 would be eligible to buy four tablets after a consultation with a pharmacist.. In 2017, the Medicines and Healthcare products Regulatory Agency enacted legislation that expanded this nationwide , allowing a particular branded formulation of Sildenafil, Viagra Connect (50mg), to be sold over the counter and without a prescription throughout the UK from early 2018. While the sale remains subject to a consultation with a pharmacist, the other restrictions from the trial have been removed, allowing customers over the age of 18 to purchase an unlimited number of pills . The decision was made, in part, to reduce online sales of counterfeit and potentially dangerous erectile dysfunction treatments.
Pfizer's patents on Viagra expired outside the US in 2012; in the US they were set to expire, but Pfizer settled litigation with each of Mylan and Teva which agreed that both companies could introduce generics in the US on 11 December 2017. In December 2017, Pfizer released its own generic version of Viagra.
Counterfeit Viagra, despite generally being cheaper, can contain harmful substances or substances that affect how Viagra works, such as blue printer ink, amphetamines, metronidazole, boric acid, and rat poison.
Pfizer's patent on sildenafil citrate expired in some member countries of the EU, Austria, Denmark, France, Germany, Ireland, Italy, The Netherlands, Spain, Sweden, the United Kingdom and Switzerland on 21 June 2013. A UK patent held by Pfizer on the use of PDE5 inhibitors (see below) as treatment of impotence was invalidated in 2000 because of obviousness; this decision was upheld on appeal in 2002.
There were 2,958,199 prescriptions for Sildenafil in 2016 in England, compared with 1,042,431 in 2006.
In 2018, Viagra Connect, a particular formulation of Sildenafil marketed by Pfizer, became available for sale without a prescription in the UK , in an attempt to widen availability and reduce demand for counterfeit products.
Sildenafil is available as a generic drug in the United States, labelled for pulmonary arterial hypertension. As of 2016[update] branded pills cost about 50 times more than generic ones. In the United States as of 2015[update] the branded 50 mg pill cost is between 25.17 and 37.88 USD.
In the United States, Pfizer received two patents for sildenafil: one for its indication to treat cardiovascular disease (marketed as Revatio) and another for its indication to treat erectile dysfunction (marketed as Viagra). The substance is the same under both trade names.
In 1992, Pfizer filed a patent covering the substance sildenafil and its use to treat cardiovascular diseases. This would be marketed as Revatio. The patent was published in 1993 and expired in 2012. The patent on Revatio (indicated for pulmonary arterial hypertension rather than erectile dysfunction) expired in late 2012. Generic versions of this low-dose form of sildenafil have been available in the U.S. from a number of manufacturers, including Greenstone, Mylan, and Watson, since early 2013. Health care providers may prescribe generic sildenafil for erectile dysfunction. However, the generic is not available in the same dosages as branded Viagra, so using dosages typically required for treating ED requires patients to take multiple pills.
In 1994, Pfizer filed a patent covering the use of sildenafil to treat erectile dysfunction. This would be marketed as Viagra. This patent was published in 2002 and will expire in 2019. Teva sued to have the latter patent invalidated, but Pfizer prevailed in an August 2011 federal district court case. An agreement with Pfizer allowed Teva to begin to provide the generic drug in December 2017.
On 8 November 2012, the Supreme Court of Canada ruled that Pfizer's patent 2,163,446 on Viagra was invalid from the beginning because the company did not provide full disclosure in its application. The decision, Teva Canada Ltd. v. Pfizer Canada Inc., pointed to section 27(3)(b) of The Patent Act which requires that disclosure must include sufficient information "to enable any person skilled in the art or science to which it pertains" to produce it. It added further: "As a matter of policy and sound statutory interpretation, patentees cannot be allowed to 'game' the system in this way. This, in my view, is the key issue in this appeal."
Teva Canada launched Novo-Sildenafil, a generic version of Viagra, on the day the Supreme Court of Canada released its decision. To remain competitive, Pfizer then reduced the price of Viagra in Canada. However, on 9 November 2012, Pfizer filed a motion for a re-hearing of the appeal in the Supreme Court of Canada, on the grounds that the court accidentally exceeded its jurisdiction by voiding the patent. Finally, on 22 April 2013, the Supreme Court of Canada invalidated Pfizer's patent altogether.
Manufacture and sale of sildenafil citrate drugs known as "generic Viagra" is common in India, where Pfizer's patent claim does not apply. Trade names include Kamagra (Ajanta Pharma), Silagra (Cipla), Edegra (Sun Pharmaceutical), Penegra (Zydus Cadila), Manly (Cooper Pharma) and Zenegra (Alkem Laboratories).
Manufacture and sale of sildenafil citrate drugs is common in China, where Pfizer's patent claim is not widely enforced.
Sildenafil was reclassified in New Zealand in 2014 so it could be bought over the counter from a pharmacist. It is thought that this reduced sales over the Internet and was safer as men could be referred for medical advice if appropriate. 
Egypt approved Viagra for sale in 2002, but soon afterwards allowed local companies to produce generic versions of the drug, citing the interests of poor people who would not be able to afford Pfizer's price.
Pfizer's patent on sildenafil citrate expired in Brazil in 2010.
- Nichols, DJ; Muirhead, GJ; Harness, JA (6 March 2002). "Pharmacokinetics of Sildenafil after Single Oral Doses in Healthy Male Subjects: Absolute Bioavailability, Food Effects and Dose Proportionality". British Journal of Clinical Pharmacology. 53: 5S–12S. doi:10.1046/j.0306-5251.2001.00027.x. PMC . PMID 11879254.
- "Viagra (sildenafil citrate) Tablets, for Oral Use. Full Prescribing Information". Pfizer Labs. Division of Pfizer, Inc., NY, NY 10017. Retrieved 5 November 2016.
- "Sildenafil Citrate". The American Society of Health-System Pharmacists. Retrieved 1 December 2014.
- "Patent US5250534 - Pyrazolopyrimidinone antianginal agents - Google Patents".
- Boolell, M; Allen, MJ; Ballard, SA; Gepi-Attee, S; Muirhead, GJ; Naylor, AM; Osterloh, IH; Gingell, C (June 1996). "Sildenafil: an Orally Active Type 5 Cyclic GMP-Specific Phosphodiesterase Inhibitor for the Treatment of Penile Erectile Dysfunction". International Journal of Impotence Research. 8 (2): 47–52. PMID 8858389.
- Vardi, M; Nini, A (24 January 2007). "Phosphodiesterase Inhibitors for Erectile Dysfunction in Patients with Diabetes Mellitus". The Cochrane Database of Systematic Reviews (1): CD002187. doi:10.1002/14651858.CD002187.pub3. PMID 17253475.
- Taylor, MJ; Rudkin, L; Bullemor-Day, P; Lubin, J; Chukwujekwu, C; Hawton, K (31 May 2013). "Strategies for Managing Sexual Dysfunction Induced by Antidepressant Medication". The Cochrane Database of Systematic Reviews (5): CD003382. doi:10.1002/14651858.CD003382.pub3. PMID 23728643. Retrieved 5 November 2016.
- Wang, RC; Jiang, FM; Zheng, QL; Li, CT; Peng, XY; He, CY; Luo, J; Liang, ZA (March 2014). "Efficacy and Safety of Sildenafil Treatment in Pulmonary Arterial Hypertension: a Systematic Review". Respiratory Medicine. 108 (3): 513–17. doi:10.1016/j.rmed.2014.01.003. PMID 24462476. Retrieved 5 November 2016.
- Roustit M, Blaise S, Allanore Y, Carpentier PH, Caglayan E, Cracowski JL (October 2013). "Phosphodiesterase-5 inhibitors for the treatment of secondary Raynaud's phenomenon: systematic review and meta-analysis of randomised trials". Annals of the Rheumatic Diseases. 72 (10): 1696–99. doi:10.1136/annrheumdis-2012-202836. PMID 23426043.
PDE-5 inhibitors appear to have significant but moderate efficacy in secondary [Raynaud's phenomenon].
- Linnemann B, Erbe M (2016). "Raynaud's phenomenon and digital ischaemia – pharmacologic approach and alternative treatment options". VASA. 45 (3): 201–12. doi:10.1024/0301-1526/a000526. PMID 27129065.
Phosphodiesterase inhibitors (e.g., sildenafil) can also improve [Raynaud's phenomenon] symptoms and ulcer healing
- "Viagra and Vision". VisionWeb. 29 October 2001. Retrieved 5 November 2016.
- "FDA Updates Labeling for Viagra, Cialis and Levitra for Rare Post-Marketing Reports of Eye Problems". U.S. Food and Drug Administration. 8 July 2005. Archived from the original on 22 October 2016. Retrieved 5 November 2016.
- Laties, AM (January 2009). "Vision Disorders and Phosphodiesterase Type 5 Inhibitors: a Review of the Evidence to Date". Drug safety. 32 (1): 1–18. doi:10.2165/00002018-200932010-00001. PMID 19132801.
- "FDA Announces Revisions to Labels for Cialis, Levitra and Viagra". U.S. Food and Drug Administration. 18 October 2007. Archived from the original on 22 October 2016. Retrieved 5 November 2016.
- Kloner, RA (26 December 2005). "Pharmacology and Drug Interaction Effects of the Phosphodiesterase 5 Inhibitors: Focus on α-Blocker Interactions". The American Journal of Cardiology. 96 (12 (Suppl. 2)): 42–46. doi:10.1016/j.amjcard.2005.07.011. PMID 16387566.
- Cheitlin, MD; Hutter, AM Jr; Brindis, RG; Ganz, P; Kaul, S; Russell, RO Jr; Zusman, RM (15 November 1999). "ACC/AHA Expert Consensus Document. Use of Sildenafil (Viagra) in Patients with Cardiovascular Disease" (PDF). Journal of the American College of Cardiology. 34 (6): 273–82. doi:10.1016/S0735-1097(98)00656-1. PMID 9935041. Retrieved 5 November 2016.
- Peterson K (21 March 2001). "Young men add Viagra to their drug arsenal". USA Today.
- Smith KM, Romanelli F (2005). "Recreational use and misuse of phosphodiesterase 5 inhibitors". J Am Pharm Assoc (2003). 45 (1): 63–72; quiz 73–75. doi:10.1331/1544345052843165. PMID 15730119.
- "Sildenafil Will Not Affect Libido - Fact!". Retrieved 15 July 2016.
- Mondaini N, Ponchietti R, Muir GH, Montorsi F, Di Loro F, Lombardi G, Rizzo M (June 2003). "Sildenafil does not improve sexual function in men without erectile dysfunction but does reduce the postorgasmic refractory time". Int. J. Impot. Res. 15 (3): 225–28. doi:10.1038/sj.ijir.3901005. PMID 12904810.
- McCambridge J, Mitcheson L, Hunt N, Winstock A (March 2006). "The rise of Viagra among British illicit drug users: 5-year survey data". Drug Alcohol Rev. 25 (2): 111–13. doi:10.1080/09595230500537167. PMID 16627299.
- Eloi-Stiven ML, Channaveeraiah N, Christos PJ, Finkel M, Reddy R (November 2007). "Does marijuana use play a role in the recreational use of sildenafil?". J Fam Pract. 56 (11): E1–4. PMID 17976333.
- "The 2007 Ig Nobel Prize Winners". Improbable Research. 4 October 2007. Retrieved 10 February 2009.
- Agostino PV, Plano SA, Golombek DA (June 2007). "Sildenafil accelerates reentrainment of circadian rhythms after advancing light schedules". Proc. Natl. Acad. Sci. U.S.A. 104 (23): 9834–39. doi:10.1073/pnas.0703388104. PMC . PMID 17519328.
- Teri Thompson; Christian Red; Michael O'Keefffe; Nathaniel Vinton (10 June 2008). "Source: Roger Clemens, host of athletes pop Viagra to help onfield performance". Daily News. Retrieved 10 February 2009.
- Busbee J (28 November 2012). "Bears' Brandon Marshall says some NFL players use Viagra … ON THE FIELD". Yahoo! Sports. Retrieved 28 November 2012.
- Venhuis BJ, de Kaste D (2006–2012). "Towards a decade of detecting new analogues of sildenafil, tadalafil and vardenafil in food supplements: a history, analytical aspects and health risks". Journal of Pharmaceutical and Biomedical Analysis. 69: 196–208. doi:10.1016/j.jpba.2012.02.014. PMID 22464558.
- Oh SS, Zou P, Low MY, Koh HL (2006). "Detection of sildenafil analogues in herbal products for erectile dysfunction". Journal of Toxicology and Environmental Health Part A. 69 (21): 1951–58. doi:10.1080/15287390600751355. PMID 16982533.
- Venhuis BJ, Blok-Tip L, de Kaste D (2008). "Designer drugs in herbal aphrodisiacs". Forensic Science International. 177 (2–3): 25–27. doi:10.1016/j.forsciint.2007.11.007. PMID 18178354.
- FDA letter to Libidus distributor
- FDA Warns Consumers About Dangerous Ingredients in "Dietary Supplements" Promoted for Sexual Enhancement
- Hidden Risks of Erectile Dysfunction "Treatments" Sold Online
- R. Baselt, Disposition of Toxic Drugs and Chemicals in Man, 9th edition, Biomedical Publications, Seal Beach, CA, 2011, pp. 1552–53. "Archived copy" (PDF). Archived from the original (PDF) on 10 September 2011. Retrieved 1 July 2011.
- Sung, B. J.; Hwang, K.; Jeon, Y.; Lee, J. I.; Heo, Y. S.; Kim, J.; Moon, J.; Yoon, J.; Hyun, Y. L.; Kim, E.; Eum, S.; Park, S. Y.; Lee, J. O.; Lee, T.; Ro, S.; Cho, J. (2003). "Structure of the catalytic domain of human phosphodiesterase 5 with bound drug molecules". Nature. 425 (6953): 98–102. doi:10.1038/nature01914. PMID 12955149.
- Webb, D.J.; Freestone, S.; Allen, M.J.; Muirhead, G.J. (4 March 1999). "Sildenafil citrate and blood-pressure-lowering drugs: results of drug interaction studies with an organic nitrate and a calcium antagonist". Am. J. Cardiol. 83 (5A): 21C–28C. doi:10.1016/S0002-9149(99)00044-2. PMID 10078539.
- "Viagra Clinical Pharmacology". RxList.com. 2008. Retrieved 20 August 2008.
- Eardley I, Ellis P, Boolell M, Wulff M (2002). "Onset and duration of action of sildenafil for the treatment of erectile dysfunction". Br J Clin Pharmacol. 53 Suppl 1: 61S–65S. doi:10.1046/j.0306-5251.2001.00034.x. PMC . PMID 11879261.
- Deveci S, Peşkircioğlu L, Aygün C, Tekin MI, Dirim A, Ozkardeş H (2004). "Sublingual sildenafil in the treatment of erectile dysfunction: faster onset of action with less dose". Int. J. Urol. 11 (11): 989–92. doi:10.1111/j.1442-2042.2004.00933.x. PMID 15509203.
- Dunn PJ (2005). "Synthesis of Commercial Phosphodiesterase(V) Inhibitors". Org Process Res Dev. 2005 (1): 88–97. doi:10.1021/op040019c.
- "Research". ABM. Abertawe Bro Morgannwg University Health Board. 4 July 2008. Archived from the original on 26 September 2008. Retrieved 6 August 2008.
Our clinicians regularly offer patients the opportunity to take part in trials of new drugs and treatments. Morriston Hospital in Swansea, was the first in the world to trial Viagra!
- Boolell M, Allen MJ, Ballard SA, Gepi-Attee S, Muirhead GJ, Naylor AM, Osterloh IH, Gingell C (June 1996). "Sildenafil: an orally active type 5 cyclic GMP-specific phosphodiesterase inhibitor for the treatment of penile erectile dysfunction". Int. J. Impot. Res. 8 (2): 47–52. PMID 8858389.
- Terrett NK, Bell AS, Brown D, Elllis P (1996). "Sildenafil (Viagra), a potent and selective inhibitor of Type 5 cGMP phosphodiesterase with utility for the treatment of male erectile dysfunction". Bioorg Med Chem Lett. 6 (15): 1819–24. doi:10.1016/0960-894X(96)00323-X.
- Ashburn, TT; Thor, KB (August 2004). "Drug repositioning: identifying and developing new uses for existing drugs". Nature Reviews. Drug Discovery. 3 (8): 673–83. doi:10.1038/nrd1468. PMID 15286734.
- Institute of Medicine (2014). Drug Repurposing and Repositioning: Workshop Summary. National Academies Press. ISBN 9780309302043.
- Kling J (1998). "From hypertension to angina to Viagra". Mod. Drug Discov. 1: 31–38. ISSN 1532-4486. OCLC 41105083.
- "Portions of the 2010 Financial Report". Retrieved 15 July 2016.
- Ciment, J (1999). "Missouri fines internet pharmacy". BMJ. British Medical Journal. 319 (7221): 1324. doi:10.1136/bmj.319.7221.1324g. PMC . PMID 10567131.
- Devine, Amy (29 September 2008). "Chemists plan to sell Viagra on the internet". Daily Record. Retrieved 30 April 2012.
- Keith A (2000). "The economics of Viagra". Health Aff (Millwood). 19 (2): 147–57. doi:10.1377/hlthaff.19.2.147. PMID 10718028.
- McGuire S (1 January 2007). "Cialis gaining market share worldwide". Medical Marketing & Media. Haymarket Media. Archived from the original on 3 January 2009. Retrieved 10 February 2009.
- Mullin, Rick (20 June 2005). "Viagra". Chemical & Engineering News. 83 (25). Retrieved 20 August 2008.
- Berenson, Alex (4 December 2005). "Sales of Impotence Drugs Fall, Defying Expectations". The New York Times. Retrieved 20 August 2008.
- "Over-the-counter Viagra piloted". BBC News. 11 February 2007. Retrieved 10 February 2009.
- "Viagra can be sold over the counter". BBC News. 28 November 2017. Retrieved 5 April 2018.
- "Viagra now available over the counter without prescription in the UK". The Independent. 27 March 2018. Retrieved 5 April 2018.
- "Pfizer to sell Viagra online, in first for Big Pharma: AP". CBS News. Retrieved 6 May 2013.
- Helfand, Carly (14 April 2015). "The coming generics threat to Pfizer's Viagra brand just got scarier". FiercePharma.
- "Viagra to go generic in 2017 according to Pfizer agreement". CBS News. 17 December 2013.
- Nocera, Joe (6 December 2017). "Sex, Drugs and That Little Blue Pill". Bloomberg. Retrieved 10 December 2017.
- Mukherjee, Sy (6 December 2017). "Why Pfizer Is About to Slash Viagra's Price in Half". Fortune. Retrieved 6 December 2017.
- "Record amount of counterfeit Viagra seized". www.bbc.com. 12 May 2016. Archived from the original on 26 December 2016. Retrieved 26 December 2016.
- Amerman, Don (22 May 2012). "Counterfeit Viagra, Cialis, Levitra: The Ultimate Guide". AccessRx. Retrieved 10 January 2017.
- Moran, Barbara (20 August 2013). "Cracking Down on Counterfeit Drugs — NOVA Next | PBS". PBS NOVA Next. Retrieved 10 January 2017.
- CNN, Gena Somra (31 August 2015). "Online pharmacies suspected of counterfeit drug sales". CNN. Retrieved 10 January 2017.
- "Actavis Launches Generic Viagra in Europe as Patents Expire". Retrieved 25 October 2013.
- Jim Edwards (21 October 2009). "What Will Happen When Viagra Goes Generic?". AccessRx.com. Retrieved 25 March 2011.
- "Is Viagra about to lose its pulling power in the UK?". The Guardian. Retrieved 13 June 2013.
- Murray-, Rosie (23 January 2002). "Viagra ruling upsets Pfizer". London: Telegraph Media Group Limited. Archived from the original on 22 August 2009. Retrieved 10 February 2009.
- "Pfizer Loses UK Battle on Viagra Patent". UroToday. Thomson Reuters. 17 June 2002. Archived from the original on 25 June 2007. Retrieved 10 February 2009.
- "Viagra prescriptions almost triple in a decade". Pharmaceutical Journal. 21 August 2017. Retrieved 8 October 2017.
- "Viagra now available over the counter without prescription in the UK". The Independent. 27 March 2018. Retrieved 5 April 2018.
- Skinner, Ginger (4 March 2016). "Yes, There Is a Way to Get Generic Viagra". Consumer Reports. Retrieved 4 May 2016.
- Langreth, Robert (29 June 2016). "Decoding Big Pharma's Secret Drug Pricing Practices". Bloomberg. Retrieved 15 July 2016.
- U.S. Patent 5,250,534
- "Pfizer's Revatio Goes Generic". Zacks Equity Research. 15 November 2012. Retrieved 5 October 2013.
- U.S. Patent 6,469,012
- "Pfizer Wins Viagra Patent Infringement Case Against Teva Pharmaceuticals". Bloomberg. 15 August 2011. Retrieved 1 April 2012.
- "Revation patent ruled invalid for lack of sound prediction and obviousness". Canadian Technology & IP Law. Stikeman Elliott. 18 June 2010. Retrieved 14 November 2012.
- "Pfizer Canada Inc. v. Ratiopharm Inc., 2010 FC 612". CanLII.
- Teva Canada Ltd. v. Pfizer Canada Inc. 2012 SCC 60 at par. 80 (8 November 2012)
- John Spears (8 November 2012). "Supreme Court ruling could lead to cheaper versions of Viagra". Toronto Star. Retrieved 14 November 2012.
- Ken Hanly (8 November 2012). "Canadian Supreme court rules Viagra patent invalid". Digital Journal. Retrieved 14 November 2012.
- "Viagra patent tossed out by Supreme Court: Decision allows generic versions of drug to be produced". CBC News. 8 November 2012. Retrieved 14 November 2012.
- "Pfizer Canada drops Viagra price after generic versions get Supreme Court green light". Financial Post. 22 November 2012. Retrieved 9 February 2013.
- "SCC Case Information, Docket No. 33951". Retrieved 14 November 2012.
- Kirk Makin (15 November 2012). "In rare move, Pfizer asks Supreme Court to reconsider ruling that killed Viagra patent". The Globe and Mail. Retrieved 15 November 2012.
- Gowling Lafleur Henderson LLP; Hélène D'Iorio (22 April 2013). "The Supreme Court of Canada holds Pfizer's Viagra patent invalid". Lexology. Retrieved 27 December 2013.
- "Viagra from the pharmacist: insight from reclassification in New Zealand". Pharmaceutical Journal. 31 January 2018. Retrieved 2 April 2018.
- Allam, Abeer (4 October 2002). "Seeking Investment, Egypt Tries Patent Laws". New York Times. Retrieved 1 April 2013.
- in-PharmaTechnologist.com. "Viagra patent expires in June, says Brazilian court". Retrieved 15 July 2016.
- Official Viagra Website
- Official Revatio Website
- prescribing information for Viagra and prescribing information for Revatio from Pfizer
- FDA Information
- MedlinePLUS information, including side effects
- U.S. National Library of Medicine: Drug Information Portal – Sildenafil
- Viagra at The Periodic Table of Videos (University of Nottingham)