[8] Rare but serious side effects include vision problems, hearing loss, and prolonged erection (priapism) that can lead to damage to the penis.

[17] The primary indication of sildenafil is treatment of erectile dysfunction (inability to sustain a satisfactory erection to complete sexual intercourse).

[19] While sildenafil improves some markers of disease in people with pulmonary arterial hypertension, it does not appear to affect the risk of death or serious side effects.

[20] 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;[9][21] these drugs have moderate efficacy for reducing the frequency and duration of vasospastic episodes.

In July 2005, the US Food and Drug Administration (FDA) updated labeling for tadalafil (Cialis), vardenafil (Levitra), and sildenafil (Viagra) to reflect a small number of post-marketing reports of sudden vision loss, while acknowledging that "...it is not possible to determine whether these oral medicines for erectile dysfunction were the cause of the loss of eyesight or whether the problem is related to other factors such as high blood pressure or diabetes, or to a combination of these problems.

"[25] A careful review of pooled data from clinical trials containing well documented information about the dose and duration of exposure to the drug for a large number of patients, yields no evidence for an increased risk of non-arteritic anterior ischemic optic neuropathy or other adverse ocular events associated with PDE-5 inhibitor use.

[4] 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.

[28] 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.

[32] Sildenafil's brand name, Viagra, is widely recognized in popular culture, and the drug's association with treating erectile dysfunction has led to its recreational use.

In one study, a 25 mg dose was shown to cause no significant change in erectile quality, but did reduce the postejaculatory refractory time.

[33] 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.

[39][40] 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.

[43] The US Food and Drug Administration (FDA) has banned numerous products claiming to be Eurycoma longifolia that, in fact, contain only analogs of sildenafil.

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.

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.

[49] 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.

This kinase is activated by cGMP and it phosphorylates multiple targets in the smooth muscle cells, namely myosin light chain phosphatase, RhoA, IP3 receptor, phospholipase C, and others.

[57] 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.

[70][71] 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).

[74] Viagra and similar prescription pharmaceuticals were promoted by images in media to the extent of becoming a cultural icon, at the time a relatively new phenomenon known to be permitted only in the United States and New Zealand and which is believed to have significantly contributed to norms regarding male sexuality.

[79][80] In 2008, the FDA forced Pfizer to remove Viva Cruiser, an advergame for Viagra, from appearing on Forbes, after the game failed to disclose risk information about the drug.

[83] In 2017, the Medicines and Healthcare products Regulatory Agency (MHRA) enacted legislation that expanded this nationwide,[84] allowing a particular branded formulation of Sildenafil, Viagra Connect (50 mg), 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.

Generic versions of this low-dose form of sildenafil have been available in the US from a number of manufacturers, including Greenstone, Mylan, and Watson, since early 2013.

[102] In Canada, Pfizer's patent 2,324,324 for Revatio (sildenafil used to treat pulmonary hypertension) was found invalid by the Federal Court in June 2010, on an application by Ratiopharm Inc.[103][104] 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.

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.

[113] In June 2013 Pfizer's patent on sildenafil citrate expired in some member countries of the European Union, including Austria, Denmark, France, Germany, Ireland, Italy, The Netherlands, Spain, Sweden, the United Kingdom, and Switzerland.

[117][118] Manufacture and sale of sildenafil citrate drugs known as "generic Viagra" is common in India, where Pfizer's patent claim does not apply.

The first document guaranteed sole production and sale of the substance until 2012, while the second gave Pfizer the exclusive use to treating erectile dysfunction with sildenafil until 2014.

The Korean Court system made a ruling against Pfizer in June 2012, allowing for the unhindered domestic production of generic prescription sildenafil.