The patent for Pfizer's Viagra sildenafil to treat erectile dysfunction expired in April 2020.
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The US Food and Drug Administration (FDA) has banned numerous products claiming to be Eurycoma longifolia that, in fact, contain only analogs of sildenafil. [44][45][46] Sellers of such fake herbals typically respond by just changing the names of their products. 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. This smooth muscle relaxation leads to vasodilation and increased inflow of blood into the spongy tissue of the penis, causing an erection.
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[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. The molecular mechanism of smooth muscle relaxation involves the enzyme CGMP-dependent protein kinase, also known as PKG. 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. [50] Overall, this results in a decrease in intracellular calcium and desensitizing proteins to the effects of calcium, engendering smooth muscle relaxation.
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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 increased penile response to sexual stimulation. [49][51] 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 sildenafil premature ejaculation major product of metabolisation by these enzymes is N-desmethylated sildenafil, which is metabolised further. 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.
| Product | Dosage | Quantity + Bonus | Price | |
|---|---|---|---|---|
| Viagra Generic | 100mg | 60 + 4 Pills | 96.36€ 91.77€ | |
| Viagra Generic | 150mg | 10 Pills | 36.73€ 34.98€ | |
| Viagra Generic | 200mg | 360 + 10 Pills | 481.98€ 459.03€ | |
| Viagra Generic | 25mg | 360 + 10 Pills | 213.68€ 203.50€ | |
| Kamagra | 100mg | 120 + 6 Pills | 330.74€ 314.99€ | |
| Viagra Generic | 100mg | 20 Pills | 45.58€ 43.41€ | |
| Viagra Generic | 100mg | 30 + 4 Pills | 60.91€ 58.01€ | |
| Kamagra | 100mg | 20 Pills | 86.09€ 81.99€ | |
| Kamagra Oral Jelly | 100mg | 90 + 8 Sachets | 303.56€ 289.10€ | |
| Viagra Generic | 25mg | 20 Pills | 37.74€ 35.94€ | |
| Kamagra Soft Tabs | 100mg | 32 Pills | 120.11€ 114.39€ | |
| Viagra Generic | 50mg | 60 + 4 Pills | 83.93€ 79.93€ | |
| Viagra Generic | 25mg | 30 + 4 Pills | 47.97€ 45.69€ |
Viagra is one of the world's most counterfeited medicines. [90][91] According to a 2012 Pfizer study, around 80% of sites claiming to sell Viagra were selling counterfeits.
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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. [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.
What Are the Available Sildenafil Dosages?
[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 women sildenafil 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. 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. 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, oral sildenafil Pfizer filed a patent covering the substance sildenafil and its use to treat cardiovascular diseases. [96] This would be marketed as Revatio.
| Side Effect | Incidence Rate | Severity | Comments |
|---|---|---|---|
| Headache | 20-30% | Mild to moderate | Usually resolves within hours |
| Flushing | 15-25% | Mild | Facial redness |
| Indigestion | 10-15% | Mild to moderate | Related to vasodilation |
| Nasal Congestion | 10% | Mild | Due to blood vessel dilation |
| Vision Changes | 2-3% | Mild to moderate | Blue tint or blurred vision |
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 US from a number of manufacturers, including Greenstone, Mylan, and Watson, since early 2013.
What causes Erectile Dysfunction?
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. When taken orally, sildenafil for erectile dysfunction results in an average time to onset of erections of 27 minutes (ranging from 12 to 70 minutes).
Study 3: Sildenafil citrate with Doxazosin
Sublingual use of sildenafil for erectile dysfunction results in an average onset of action of 15 minutes and lasting for an average of 40 minutes. The preparation steps for synthesis of sildenafil are:[54] Methylation of 3-propylpyrazole-5-carboxylic acid ethyl ester with hot dimethyl sulfate Hydrolysis with aqueous sodium hydroxide (NaOH) to free acid Carboxamide formation with refluxing thionyl chloride/NH4OH Reduction of nitro group to amino group Sildenafil (compound UK-92,480) was synthesized by a group of pharmaceutical chemists led by Simon Campbell[55] 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). [56] The first clinical trials were conducted in Morriston Hospital in Swansea. [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.
Allergy warning
[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). Numerous sites on the Internet offer Viagra for sale after an "online consultation", often a simple web questionnaire. [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.
- Sildenafil 50mg is a phosphodiesterase inhibitor for ED treatment.
- The medication is not a cure but helps achieve an erection on demand.
- Avoid combining sildenafil with recreational drugs or alcohol.
- It is important to follow the timing recommended by your doctor.
- The medication may cause a temporary decrease in blood pressure.
- Use with caution in individuals with kidney or liver impairment.
- Always read the patient information leaflet that accompanies the medicine.
- Regular medical check-ups are advised when using sildenafil long-term.
In 1994, Pfizer filed a patent covering the use of sildenafil to treat erectile dysfunction.
| Medication Class | Interaction Type | Effect | Notes |
|---|---|---|---|
| Nitrates | Additive hypotensive effect | Severe blood pressure drop | Contraindicated |
| Alpha-blockers | Enhanced vasodilation | Risk of hypotension | Use with caution |
| CYP3A4 Inhibitors | Increased sildenafil levels | Higher risk of side effects | Dose adjustment may be required |
| Other PDE5 inhibitors | Additive effect | Increased efficacy or side effects | Not recommended to combine |
[99] This would be marketed as Viagra.
- Sildenafil citrate 50mg may cause priapism, a prolonged erection needing medical attention.
- Use with caution in men taking blood pressure medications.
- Check for contraindications: heart disease, stroke history, or liver issues.
- Avoid taking sildenafil with fatty meals to improve absorption time.
- The drug's effectiveness can be reduced if taken with certain drugs.
- Patients should inform their doctor of all medications they are taking.
- The medication does not protect against sexually transmitted infections.
- Follow the prescribed dosage schedule strictly to avoid adverse reactions.
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.
- Sildenafil citrate 50mg can be used in combination therapy for ED.
- It is vital to follow your doctor’s exact dose and timing instructions.
- Do not take sildenafil if you have a history of allergic reactions.
- Exercise caution if you experience chest pains while on the medication.
- Keep track of any side effects experienced and report them.
- Avoid physical activity immediately after taking the tablet if feeling dizzy.
- The medication does not protect against STDs; use barrier protection.
- Always inform your healthcare provider of all ongoing treatments and medications.
[100] An agreement with Pfizer allowed Teva to begin to provide the generic drug in December 2017.
- The tablets are typically taken about 30 minutes to 1 hour before sex.
- Effects may vary depending on individual health and other medications.
- Patients should avoid heavy or fatty meals before taking sildenafil.
- Do not share your medication with others; it is prescribed specifically for you.
- Sildenafil 50mg is not intended for use in women or children.
- Seek immediate medical attention if you experience sudden hearing or vision loss.
- Do not use sildenafil if you have severe cardiovascular disease.
- Proper hydration is recommended while taking the medication.
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).
| Ingredient | Quantity per Tablet | Purpose | Notes |
|---|---|---|---|
| Sildenafil Citrate | 50 mg | Active pharmaceutical ingredient | Main component |
| Microcrystalline Cellulose | 30 mg | Filler and binder | Ensures tablet integrity |
| Croscarmellose Sodium | 10 mg | Disintegrant | Promotes dissolution |
| Magnesium Stearate | 2 mg | Lubricant | Prevents sticking during compression |
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.
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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. Seven of these companies are based in India.