The Chemical Structure of Sildenafil Citrate

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.

Country	Approval Status	Regulatory Agency	Notes
USA	Approved	FDA	For ED and pulmonary hypertension
European Union	Approved	EMA	Similar indications
India	Approved	CDSCO	Generic versions available
Australia	Approved	TGA	Prescription required

[117][118][119] 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.

Property	Description	Value	Units
Molecular Weight	The mass of one mole of sildenafil citrate	474.58	g/mol
Chemical Formula	Molecular composition	C22H30N6O4S
Solubility	Water solubility	Slightly soluble	at 25°C
pH of Solution	Typical pH when dissolved in water	3.8 - 4.2
Melting Point	The temperature at which it melts	196 - 198	°C

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

• Sildenafil is unsuitable for individuals with severe liver disease.
• Patients should avoid alcohol consumption during treatment.
• Some side effects may resolve on their own within hours.
• Persistent side effects should prompt medical consultation.
• The drug does not cure ED but manages symptoms.
• Lifestyle changes can also improve erectile function.
• Follow your healthcare provider’s instructions precisely.

Brand names include Kamagra (Ajanta Pharma), Silagra (Cipla), Edegra (Sun Pharmaceutical), Penegra (Zydus Cadila), Manly (Cooper Pharma) and Zenegra (Alkem Laboratories).

• Sildenafil citrate was first approved by the FDA in 1998.
• Its brand name is Viagra, but generic versions are available.
• The drug is available in tablets of various strengths, typically 25-100 mg.
• It can be prescribed for both men and in specific cases, women.
• Proper dosage varies based on health status and response.
• Use caution in patients with heart problems or low blood pressure.
• It is not a hormone or a testosterone booster.

Sildenafil was reclassified in New Zealand in 2014 so it could be bought over the counter from a pharmacist.

Mechanism of action

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. It is thought that this reduced sales over the Internet and was safer as males could be referred for medical advice if appropriate.

Product	Dosage	Quantity + Bonus	Price
Viagra Generic	100mg	270 + 10 Pills	270.47€ 257.59€	Buy
Kamagra Oral Jelly	100mg	21 Sachets	95.92€ 91.35€	Buy
Viagra Generic	150mg	20 Pills	55.02€ 52.40€	Buy
Viagra Generic	200mg	10 Pills	40.96€ 39.01€	Buy
Kamagra Polo	100mg	272 + 12 Pills	622.94€ 593.28€	Buy
Viagra Generic	25mg	120 + 6 Pills	125.56€ 119.58€	Buy
Viagra Generic	25mg	20 Pills	37.74€ 35.94€	Buy
Kamagra Oral Jelly	100mg	90 + 8 Sachets	303.56€ 289.10€	Buy
Kamagra Polo	100mg	120 + 6 Pills	327.15€ 311.57€	Buy
Kamagra Soft Tabs	100mg	60 + 4 Pills	180.59€ 171.99€	Buy
Viagra Generic	100mg	180 + 8 Pills	199.37€ 189.88€	Buy
Kamagra Soft Tabs	100mg	120 + 6 Pills	311.78€ 296.93€	Buy
Viagra Generic	25mg	90 + 6 Pills	105.03€ 100.03€	Buy
Viagra Generic	50mg	30 + 4 Pills	54.26€ 51.68€	Buy

In 1999 South Korea granted two patents to Pfizer related to sildenafil.

Frequently asked questions

[109][110][111] To remain competitive, Pfizer then reduced the price of Viagra in Canada. [112] However, on 9 November 2012, Pfizer filed a motion for a re-hearing of the appeal in the Supreme Court of Canada,[113] on the grounds that the court accidentally exceeded its jurisdiction by voiding the patent. [114] Finally, on 22 April 2013, the Supreme Court of Canada invalidated Pfizer's patent altogether. Manufacture and sale of sildenafil citrate drugs 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. 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.

MeSH terms

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 US from a number of manufacturers, including Greenstone, Mylan, and Watson, since early 2013. [97] Health care providers may prescribe generic sildenafil for erectile dysfunction. [98] For a time, the generic was not available in the same dosages as branded Viagra, so using dosages typically required for treating ED required patients to take multiple pills. In 1994, Pfizer filed a patent covering the use of sildenafil to treat erectile dysfunction.

Incidence not known

[99] This would be marketed as Viagra. This patent was published in 2002 and expired in 2019. Teva sued to have the latter patent invalidated, but Pfizer prevailed in an August 2011 federal district court case. [100] An agreement with Pfizer allowed Teva to begin to provide the generic drug in December 2017. 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).

Common Adverse Effects

The substance is the same under both brand names. Sildenafil is available as a generic drug in the United States, labeled for pulmonary arterial hypertension,[98] and to treat erectile dysfunction, as the patent expired in April 2020. In the US, Revatio and sildenafil citrate 100 mg Viagra are marketed by Viatris after Upjohn was spun off from Pfizer. 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.[106][107] 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. [123] In 2011 Hanmi Pharmaceutical and CJ CheilJedang launched a suit against the exclusive use patent.

Condition	Description	Recommended Dosage	Notes
Erectile Dysfunction (ED)	Difficulty achieving or maintaining an erection	50 mg before activity	Can be adjusted based on response
Pulmonary Arterial Hypertension	High blood pressure in lungs	20 mg three times daily	Prescribed under medical supervision
Off-label uses	Other potential uses	Varies	Use under healthcare provider guidance

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

Dosing & Uses

[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. [58][59] Pfizer therefore decided to market it for erectile dysfunction, rather than for angina; this decision became an often-cited example of drug repositioning. [60][61] 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. [62] It soon became a great success: annual sales of Viagra peaked in 2008 at US$1.934 billion. 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).

Side effect details

Numerous sites on the Internet offer Viagra for sale after an "online consultation", often a simple web questionnaire. Viagra and other products for sexual dysfunction, termed sexuopharmaceuticals, proliferated new types of specialised marketing for such products. [66] 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. [67] One author notes that although the effect of Viagra is only limited to penile blood vessels, advertisements routinely use imagery of couples hugging, smiling and dancing, with the author claiming that pharmaceutical companies were deceptive in the use of such advertisements. In 2000, Viagra sales accounted for 92% of the global market for prescribed erectile dysfunction pills.

Serious Side Effects

[69] By 2007, Viagra's global share had plunged to about 50%[70] 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. [71][72] 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. 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. Males between the ages of 30 and 65 would be eligible to buy four tablets after a consultation with a pharmacist. [75] In 2017, the Medicines and Healthcare products Regulatory Agency (MHRA) enacted legislation that expanded this nationwide,[76] 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. In 2012, Viagra lost its position as the top selling erectile dysfunction treatment in South Korea.

• Sildenafil is available in most countries through prescription only.
• It is used worldwide, with varying approval statuses.
• FDA and EMA have rigorous testing standards for sildenafil.
• Using the drug responsibly can lead to successful treatment outcomes.
• Combining sildenafil with other medications requires medical supervision.
• Be aware of the potential for drug interactions with heart or blood pressure meds.
• Always consult a healthcare professional for personalized advice.

This development was credited largely "due to the introduction of generic products." Generic sildenafil became publicly available in May.

IV Compatibility

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. In May 2013, Pfizer, which manufactures Viagra, told the Associated Press they will begin selling the drug directly to people on its website. 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 in December 2017. [79][80] In December 2017, Pfizer released its own generic version of Viagra. As of 2018[update], the US Food and Drug Administration has approved fifteen drug manufacturers to market generic sildenafil in the United States.

Brand Names

Seven of these companies are based in India. The patent for Pfizer's Viagra sildenafil to treat erectile dysfunction expired in April 2020. In 2026 a chocolate syrup called Boner Bears from Lockout Supplements was recalled through the FDA because of it containing Sildenafil as an undisclosed ingredient. Counterfeit Viagra, despite generally being cheaper,[88] can contain harmful substances or substances that affect how Viagra works, such as blue printer ink, amphetamines, metronidazole, boric acid, and rat poison. Viagra is one of the world's most counterfeited medicines.

Viagra and other medications

[90][91] According to a 2012 sildenafil 10mg Pfizer study, around 80% of sites claiming to sell Viagra were selling counterfeits. An October 2023 release stated that erectile dysfunction medicines were the most seized drugs by the Interpol[92] accounting for 22% of seizures. In 1992, Pfizer filed a patent covering the substance sildenafil and its use to treat cardiovascular diseases. [96] This would be marketed as Revatio. The patent was published in 1993 and expired in 2012. Sales of PalPal by Hanmi Pharmaceuticals totalled ₩22 billion or about 86% the market share of Viagra that year. [125] By 2017, there were over 50 generic sildenafil pills available. During that year Viagra sales slumped to 38% that of Palpal. There were 2,958,199 prescriptions for Sildenafil in 2016 in England, compared with 1,042,431 in 2006.

• Sildenafil citrate is manufactured by several pharmaceutical companies.
• Its chemical formula is C22H30N6O4S.
• It has a half-life of approximately 4 hours in the body.
• The drug enhances nitric oxide activity in the corpus cavernosum.
• It’s important to follow prescribed dosing to avoid adverse effects.
• It is classified as a phosphodiesterase type 5 inhibitor.
• Sildenafil was initially researched for angina before ED use.