|Systematic (IUPAC) name|
|Licence data||EMA: , US FDA:|
|Pregnancy cat.||B (US)|
|Legal status||℞ Prescription only|
|Metabolism||Hepatic (mostly CYP3A4, also CYP2C9)|
|Half-life||3 to 4 hours|
|Excretion||Fecal (80%) and renal (around 13%)|
|PDB ligand ID||VIA (, )|
|Mol. mass||base: 474.6 g/mol|
|(what is this?)|
Sildenafil citrate, sold as Viagra, Revatio and under various other trade names, is a drug used to treat erectile dysfunction and pulmonary arterial hypertension (PAH). It was originally discovered by Pfizer scientists Andrew Bell, David Brown, and Nicholas Terrett. It acts by inhibiting cGMP-specific phosphodiesterase type 5 (PDE5), an enzyme that promotes degradation of cGMP, which regulates blood flow in the penis. Since becoming available in 1998, sildenafil has been the prime treatment for erectile dysfunction; its primary competitors on the market are tadalafil (Cialis) and vardenafil (Levitra).
- 1 Medical uses
- 2 Adverse effects
- 3 Non-medical use
- 4 Detection in biological fluids
- 5 Mechanism of action
- 6 Chemical synthesis
- 7 History
- 8 References
- 9 External links
The primary indication of sildenafil is treatment of erectile dysfunction (inability to sustain a satisfactory erection to complete intercourse). Its use is now standard treatment for erectile dysfunction in all settings, including diabetes.
People on antidepressants may experience sexual dysfunction, either as a result of their illness or as a result of their treatment. A 2003 study showed that sildenafil improved sexual function in men in this situation. Following up reports from 1999, the same researchers found that sildenafil improved sexual function in female patients on antidepressants as well.
As well as erectile dysfunction, sildenafil citrate is also effective in the rare disease pulmonary arterial hypertension (PAH). It relaxes the arterial wall, leading to decreased pulmonary arterial resistance and pressure. This, in turn, reduces the workload of the right ventricle of the heart and improves symptoms of right-sided heart failure. Because PDE5 is primarily distributed within the arterial wall smooth muscle of the lungs and penis, sildenafil acts selectively in both these areas without inducing vasodilation in other areas of the body. Pfizer submitted an additional registration for sildenafil to the United States Food and Drug Administration (FDA), and sildenafil was approved for this indication in June 2005. The preparation is named Revatio, to avoid confusion with Viagra, and the 20 milligram tablets are white and round. Sildenafil joins bosentan and prostacyclin-based therapies for this condition.
Sildenafil has been shown to be useful for the prevention and treatment of high-altitude pulmonary edema associated with altitude sickness such as that suffered by mountain climbers. While this effect has only recently been discovered, sildenafil is already becoming an accepted treatment for this condition, in particular in situations where the standard treatment of rapid descent has been delayed for some reason.
Viagra pills are blue and diamond-shaped with the word "Pfizer" engraved on one side, and "VGR xx" (where xx stands for "25", "50" or "100", the dose of that pill in milligrams) engraved on the other. It is taken not more than once per day between 30 minutes and 4 hours prior to sexual intercourse.
The dosage for pulmonary arterial hypertension (Revatio) is three times a day. Revatio pills are white, round, film-coated tablets imprinted with "RVT 20" embossed on one side.
In clinical trials, the most common adverse effects of sildenafil use included headache, flushing, dyspepsia, 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 nonarteritic anterior ischemic optic neuropathy. Rare but serious adverse effects found through postmarketing surveillance include priapism, severe hypotension, myocardial infarction (heart attack), ventricular arrhythmias, stroke, increased intraocular pressure, and sudden hearing loss. As a result of these postmarketing reports, 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 patients that are also taking protease inhibitors for the treatment of HIV. Protease inhibitors inhibit the metabolism of sildenafil, effectively multiplying the plasma levels of sildenafil, increasing the incidence and severity of side effects. It is recommended that patients using protease inhibitors 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.
- When taking nitric oxide donors, organic nitrites and nitrates, such as glyceryl trinitrate (nitroglycerin), sodium nitroprusside, amyl nitrite ("poppers")
- In men for whom sexual intercourse is inadvisable due to cardiovascular risk factors
- Severe hepatic impairment (decreased liver function)
- Severe impairment in renal function
- Hypotension (low blood pressure)
- Recent stroke or heart attack
- Hereditary degenerative retinal disorders (including genetic disorders of retinal phosphodiesterases)
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 that 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 post-ejaculatory 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 Viagra helps treat jet lag recovery in hamsters. Their research was published in the Proceedings of the National Academy of Sciences.
Use in sports
Professional athletes have been documented using sildenafil, believing the opening of their blood vessels will enrich their muscles. In turn, they believe that it will enhance their performance.
Israeli and Australian researchers discovered that 1 mg of the drug dissolved in a vase of water can extend the shelf life of cut flowers, making them stand up straight for up to a week beyond their natural life span. The drug also slows down plant ripening; tests were done strawberries, legumes, roses, carnations, broccoli, and other perishables. Viagra increases the vase life of the flowers by slowing the breakdown of cGMP by cGMP-specific phosphodiesterase type 5. The Viagra acts on the cGMP in a fashion similar to nitric oxide (which also slows down the ripening process), but was found to be easier to use with cut flowers.
Acetildenafil is a structural analog of sildenafil, one of the PDE5 inhibitors found in a number of "herbal" aphrodisiac products sold over-the-counter. This class of analogs has not undergone any of the rigorous testing that drugs like sildenafil have passed, and, thus, has an unknown side-effect profile. 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 analogues of illegal drugs of abuse, rather than analogues of prescription medicines. However, at least one court case has resulted in a product being taken off the market.
The United States FDA has banned numerous products claiming to be Eurycoma longifolia that, in fact, contain only analogs of sildenafil. Peddlers 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 quantitated 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 overdosage.
Mechanism of action
The mechanism of action of sildenafil involves the protection of 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 (approximately 80% of administered oral dose) and to a lesser extent in the urine (approximately 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.
The preparation steps for synthesis of sildenafil are as follows:
- Methylation of 3-propylpyrazole-5-carboxylic acid ethyl ester with hot dimethyl sulfate
- Hydrolysis with aqueous NaOH to free acid
- Nitration with oleum/fuming nitric acid
- Carboxamide formation with refluxing thionyl chloride/NH4OH
- Reduction of nitro group to amino
- 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 that the drug had little effect on angina, but that it could induce marked penile erections. Pfizer therefore decided to market it for erectile dysfunction, rather than for angina. The drug was patented in 1996, approved for use in erectile dysfunction by the FDA on March 27, 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 million.
The British press portrayed Peter Dunn and Albert Wood as the inventors of the drug, however only Andrew Bell, David Brown, and Nicholas Terrett are listed on the original composition of matter patent.
Marketing and sales
Even though sildenafil is available only by prescription from a doctor, it was advertised directly to consumers on U.S. 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 that 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", "Blue Diamond" as well as various other nicknames.
In 2000, Viagra sales accounted for 92 percent of the global market for prescribed erectile dysfunction pills. By 2007, Viagra's global share had plunged to about 50 percent 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 30 and 65 would be eligible to buy four tablets after a consultation with a pharmacist.
In 2008/9, reports emerged of police and other authorities in Malaysia seizing sachets of coffee laced with sildenafil. Numerous brands tested were found to contain it, and were removed from sale.
Patent issues and expirations
Pfizer's patent on sildenafil citrate expired in some member countries of the EU, Austria, Denmark, France, Germany, Ireland, Italy, The Netherlands, Spain, Sweden, United Kingdom as well as 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.
In 1992 Pfizer filed a patent covering the substance sildenafil and its use to treat cardiovascular diseases. This patent was published in 1993 and expired in 2012. In 1994 Pfizer filed a patent covering the use of sildenafil to treat erectile dysfunction. 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.
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. There is no legal barrier to doctors prescribing this form of sildenafil "off label" for erectile dysfunction, although the dosage typically required for treating ED requires patients to take multiple pills.
On November 8, 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 November 9, 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 April 22, 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), and Zenegra (Alkem Laboratories).
Manufacture and sale of sildenafil citrate drugs known as "generic viagra" is common in China, where Pfizer's patent claim is not widely enforced.
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.
- "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". Int. J. Impot. Res. 8 (2): 47–52. PMID 8858389.
- Vardi M, Nini A (2007). "Phosphodiesterase inhibitors for erectile dysfunction in patients with diabetes mellitus". In Vardi, Moshe. Cochrane Database Syst Rev (1): CD002187. doi:10.1002/14651858.CD002187.pub3. PMID 17253475.
- Nurnberg HG, Hensley PL, Gelenberg AJ, Fava M, Lauriello J, Paine S (January 2003). "Treatment of antidepressant-associated sexual dysfunction with sildenafil: a randomized controlled trial". JAMA 289 (1): 56–64. doi:10.1001/jama.289.1.56. PMID 12503977.
- Nurnberg HG, Hensley PL, Lauriello J, Parker LM, Keith SJ (August 1999). "Sildenafil for women patients with antidepressant-induced sexual dysfunction". Psychiatr Serv 50 (8): 1076–8. PMID 10445658.
- Nurnberg HG, Hensley PL, Heiman JR, Croft HA, Debattista C, Paine S (2008). "Sildenafil Treatment of Women With Antidepressant-Associated Sexual Dysfunction". JAMA 300 (4): 395–404. doi:10.1001/jama.300.4.395. PMID 18647982.
- "FDA Approves Pfizer's Revatio as Treatment for Pulmonary Arterial Hypertension" (Press release). Pfizer. June 6, 2005. Archived from the original on August 28, 2005.
- Richalet JP, Gratadour P, Robach P, et al. (2005). "Sildenafil inhibits altitude-induced hypoxemia and pulmonary hypertension". Am. J. Respir. Crit. Care Med. 171 (3): 275–81. doi:10.1164/rccm.200406-804OC. PMID 15516532.
- Perimenis P (2005). "Sildenafil for the treatment of altitude-induced hypoxaemia". Expert Opin Pharmacother 6 (5): 835–7. doi:10.1517/146565126.96.36.1995. PMID 15934909.
- Fagenholz PJ, Gutman JA, Murray AF, Harris NS (2007). "Treatment of high altitude pulmonary edema at 4240 m in Nepal". High Alt. Med. Biol. 8 (2): 139–46. doi:10.1089/ham.2007.3055. PMID 17584008.
- "Pill Identifier". Drugs.com. Retrieved 2009-02-10. "This site is intended for viewing by the USA audience only. If you are in another country, local laws may not permit access to the medical information contained in this site."
- "Viagra Prescribing Information" (PDF). Pfizer. October 2007. Archived from the original on 14 November 2012. Retrieved 14 November 2012.
- "Viagra and vision". VisionWeb. 29 October 2001. Retrieved 2009-02-10.
- "FDA Updates Labeling for Viagra, Cialis and Levitra for Rare Post-Marketing Reports of Eye Problems". United States Food and Drug Administration. 8 July 2005. Archived from the original on February 23, 2008. Retrieved 2009-02-10.
- Laties AM (2009). "Vision disorders and phosphodiesterase type 5 inhibitors: a review of the evidence to date". Drug Saf 32 (1): 1–18. doi:10.2165/00002018-200932010-00001. PMID 19132801.
- "FDA Announces Revisions to Labels for Cialis, Levitra and Viagra". United States Food and Drug Administration. 18 October 2007. Archived from the original on 11 July 2007. Retrieved 10 February 2009.
- "Viagra (sildenafil citrate) tablets". page 29: Pzifer. October 2007. Retrieved 2009-10-25.
- Kloner RA (2005). "Pharmacology and drug interaction effects of the phosphodiesterase 5 inhibitors: focus on alpha-blocker interactions". Am J Cardiol 96 (12B): 42M–46M. 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 (1999). "ACC/AHA expert consensus document. Use of sildenafil (Viagra) in patients with cardiovascular disease. American College of Cardiology/American Heart Association". Journal of the American College of Cardiology 33 (1): 273–82. doi:10.1016/S0735-1097(98)00656-1. PMID 9935041.
- Peterson K (2001-03-21). "Young men add Viagra to their drug arsenal". USAToday.
- Smith KM, Romanelli F (2005). "Recreational use and misuse of phosphodiesterase 5 inhibitors". J Am Pharm Assoc (2003) 45 (1): 63–72; quiz 73–5. doi:10.1331/1544345052843165. PMID 15730119.
- Sildenafil Will Not Affect Libido - Fact!
- 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): 225–8. 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): 111–3. 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 2009-02-10.
- 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): 9834–9. doi:10.1073/pnas.0703388104. PMC 1887561. PMID 17519328.
- Teri Thompson, Christian Red, Michael O'Keefffe, and Nathaniel Vinton (10 June 2008). "Source: Roger Clemens, host of athletes pop Viagra to help onfield performance". Daily News (Daily News). Retrieved 2009-02-10.
- Busbee J (2012-11-28). "Bears’ Brandon Marshall says some NFL players use Viagra … ON THE FIELD". Yahoo! Sports. Retrieved 2012-11-28.
- Siegel-Itzkovich J (July 1999). "In brief: Viagra makes flowers stand up straight". BMJ 319 (7205): 274B. doi:10.1136/bmj.319.7205.274a. PMC 1126921. PMID 10426724.
- "Prolongation of the shelf life of fruits and flowers". Biology Department, University of Hamburg. Retrieved 28 November 2012.
- Oh SS, Zou P, Low MY, Koh HL. Detection of sildenafil analogues in herbal products for erectile dysfunction (2006). "Detection of sildenafil analogues in herbal products for erectile dysfunction". Journal of Toxicology and Environmental Health Part A 69 (21): 1951–1958. 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. 1552–1553. http://www.biomedicalpublications.com/dt9.pdf
- 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. (March 4, 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 2008-08-20.
- 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!"
- 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): 1819–1824. doi:10.1016/0960-894X(96)00323-X.
- Kling J (1998). "From hypertension to angina to Viagra". Mod. Drug Discov. 1: 31–38. ISSN 1532-4486. OCLC 41105083.
- Viagra sales peak at $1,934m in 2008
- Ciment, J (1999). "Missouri fines internet pharmacy". BMJ (British Medical Journal) 319 (7221): 1324. doi:10.1136/bmj.319.7221.1324g. PMC 1174637. PMID 10567131.
- Devine, Amy (September 29, 2008). "Chemists plan to sell Viagra on the internet". Daily Record. Retrieved 2012-04-30.
- "Urban Dictionary: Vitamin V". Urban Dictionary. January 16, 2009. Retrieved 2013-12-27.
- 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 (2007-01-01). "Cialis gaining market share worldwide". Medical Marketing & Media. Haymarket Media. Archived from the original on 2009-01-03. Retrieved 2009-02-10.
- Mullin, Rick (June 20, 2005). "Viagra". Chemical & Engineering News 83 (25). Retrieved 2008-08-20.
- Berenson, Alex (December 4, 2005). "Sales of Impotence Drugs Fall, Defying Expectations". The New York Times. Retrieved 2013-12-27.
- "Over-the-counter Viagra piloted". BBC News. BBC News. 11 February 2007. Retrieved 2009-02-10.
- "Pfizer to sell Viagra online, in first for Big Pharma: AP". CBS News. Retrieved 6 May 2013.
- Malaysian authorities seize 'Viagra coffee', report, phys.org, 5 July 2009, accessed 8 February 2014
- "Actavis Launches Generic Viagra in Europe as Patents Expire". Retrieved 2013-10-25.
- Jim Edwards (October 21, 2009). "What Will Happen When Viagra Goes Generic?". AccessRx.com. Retrieved 2013-12-27.
- "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.
- U.S. Patent 5,250,534
- U.S. Patent 6,469,012
- "Pfizer Wins Viagra Patent Infringement Case Against Teva Pharmaceuticals". Bloomberg. August 15, 2011. Retrieved 2012-04-01.
- "Pfizer's Revatio Goes Generic". Zacks Equity Research. November 15, 2012. Retrieved 2013-10-05.
- "Revation patent ruled invalid for lack of sound prediction and obviousness". Canadian Technology & IP Law. Stikeman Elliott. 2010-06-18. Retrieved 2012-11-14.
- "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 (2012-11-08). "Supreme Court ruling could lead to cheaper versions of Viagra". The [Toronto] Star. Retrieved 2012-11-14.
- Ken Hanly (2012-11-08). "Canadian Supreme court rules Viagra patent invalid". Digital Journal. Retrieved 2012-11-14.
- "Viagra patent tossed out by Supreme Court: Decision allows generic versions of drug to be produced". CBC News. 2012-11-08. Retrieved 2012-11-14.
- "Pfizer Canada drops Viagra price after generic versions get Supreme Court green light". Financial Post. 2012-11-22. Retrieved 2013-12-27.
- "SCC Case Information, Docket No. 33951". Retrieved 2012-11-14.
- Kirk Makin (2012-11-15). "In rare move, Pfizer asks Supreme Court to reconsider ruling that killed Viagra patent". The Globe and Mail. Retrieved 2012-11-15.
- Gowling Lafleur Henderson LLP, Hélène D'Iorio (2013-04-22). "The Supreme Court of Canada holds Pfizer’s Viagra patent invalid". Lexology. Retrieved 2013-12-27.
- Allam, Abeer (October 4, 2002). "Seeking Investment, Egypt Tries Patent Laws". New York Times. Retrieved 2013-12-27.
- Viagra patent expires in June, says Brazilian court
|Find more about Viagra at Wikipedia's sister projects|
|Definitions and translations from Wiktionary|
|Media from Commons|
|Quotations from Wikiquote|
|Source texts from Wikisource|
|Textbooks from Wikibooks|
|Learning resources from Wikiversity|
- Official Viagra Website
- Official Viagra UK 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)