Sildenafil

Sildenafil belongs to a class of medications called phosphodiesterase type 5 (PDE5) inhibitors ^[1]. To understand how it works, it helps to know how an erection normally occurs.

When a man becomes sexually aroused, nerve signals trigger the release of nitric oxide (NO) in the erectile tissue of the penis (the corpus cavernosum) ^{[1, 10]}. Nitric oxide activates an enzyme called guanylate cyclase, which increases levels of a chemical called cyclic guanosine monophosphate (cGMP). cGMP causes the smooth muscle cells in the blood vessel walls of the penis to relax, allowing blood to flow in and fill the erectile tissue, producing an erection ^[1].

Normally, an enzyme called PDE5 breaks down cGMP, which eventually causes the erection to subside ^[1]. In men with erectile dysfunction, this breakdown may happen too quickly or cGMP production may be insufficient, making it difficult to achieve or maintain an erection.

Sildenafil works by blocking PDE5, preventing the breakdown of cGMP ^{[1, 10]}. This allows cGMP to accumulate to higher levels and persist longer, enhancing the natural erectile response to sexual stimulation. Importantly, sildenafil does not cause an erection on its own — sexual arousal and nitric oxide release are still required to initiate the process ^{[1, 4]}.

For pulmonary arterial hypertension (PAH), the same mechanism applies in the lungs ^[2]. PDE5 is also present in pulmonary vascular smooth muscle. By inhibiting PDE5 in the pulmonary arteries, sildenafil causes vasodilation, reducing pulmonary artery pressure and improving exercise capacity, as demonstrated in the landmark SUPER-1 trial ^[5].

Sildenafil is absorbed rapidly after oral administration, reaching peak blood levels in 30-120 minutes (median 60 minutes) ^[1]. Its effects last approximately 4-5 hours. The original NEJM trial by Goldstein et al. established sildenafil as a breakthrough treatment for erectile dysfunction ^[4].

For erectile dysfunction, you should take sildenafil approximately 30-60 minutes before you plan to engage in sexual activity ^[1]. The medication typically begins working within 30 minutes, though some men notice effects as early as 15-20 minutes ^{[1, 4]}.

When the medication takes effect, you will not get an automatic erection. Sexual stimulation is required ^{[1, 4]}. When aroused, you may find it easier to achieve and maintain an erection. The effects of sildenafil last approximately 4-5 hours, giving you a window during which erections may be easier to achieve ^[1].

Common initial side effects include headache (16%), facial flushing (10%), and nasal congestion ^{[1, 4]}. These are generally mild and result from the blood vessel-relaxing effects of the medication. They typically improve with continued use.

Some men notice a slight blue tinge to their vision or increased sensitivity to light ^{[1, 8]}. This is caused by mild inhibition of PDE6 in the retina and is temporary, resolving as the drug clears your system. The FDA has revised labels to include warnings about rare cases of non-arteritic anterior ischemic optic neuropathy (NAION) and sudden sensorineural hearing loss ^{[7, 8]}.

Do not take sildenafil with a high-fat meal, as fat significantly delays absorption and may reduce effectiveness ^[1]. A light meal or empty stomach is optimal.

The recommended starting dose for most men is 50 mg ^[1]. Your doctor may adjust this to 25 mg or 100 mg based on effectiveness and tolerability. Do not take more than one dose per day ^[1]. If you experience an erection lasting more than 4 hours (priapism), seek emergency medical attention immediately, as this can cause permanent damage ^[1].

High-fat meals significantly affect sildenafil absorption ^[1]. Taking sildenafil with a high-fat meal delays the onset of action by about 60 minutes and reduces peak blood levels by approximately 29% ^[1]. For optimal effectiveness, take sildenafil on an empty stomach or after a light, low-fat meal.

Alcohol in moderate to heavy amounts can worsen the blood pressure-lowering effect of sildenafil, increasing the risk of dizziness, lightheadedness, fainting, and headache ^{[1, 12]}. Alcohol also independently impairs erectile function ^[6]. Limit alcohol to 1-2 drinks if you plan to take sildenafil.

Grapefruit juice inhibits CYP3A4 and can moderately increase sildenafil blood levels ^{[1, 12]}. While not contraindicated, large amounts of grapefruit juice may increase the risk of side effects.

Critical interaction with nitrates: Sildenafil is absolutely contraindicated with any form of nitrate medication (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate, amyl nitrite) ^{[1, 14]}. The combination can cause severe, potentially fatal hypotension. Do not take sildenafil within 24 hours of any nitrate medication.

Erectile dysfunction (Viagra) [1]: - Standard dose: 50 mg taken approximately 1 hour before sexual activity - Dose range: 25-100 mg - Maximum frequency: Once per day - Starting dose adjustments: Begin at 25 mg for patients >65 years, hepatic impairment, severe renal impairment (CrCl <30 mL/min), or concurrent CYP3A4 inhibitors ^[1]

Pulmonary arterial hypertension (Revatio) [2, 5]: - Oral: 5 mg or 20 mg three times daily, approximately 4-6 hours apart - IV: 2.5 mg or 10 mg IV bolus three times daily (for patients temporarily unable to take oral) - Do not exceed recommended PAH doses

Available forms [1, 2]: - Viagra tablets: 25 mg, 50 mg, 100 mg - Revatio tablets: 20 mg - Revatio oral suspension: 10 mg/mL - Revatio injection: 10 mg/12.5 mL

Important notes: Sildenafil for ED is taken as needed, not on a daily schedule ^[1]. For PAH, it is taken on a fixed three-times-daily schedule ^[2]. Do not switch between Viagra and Revatio formulations without medical guidance, as the dosing is completely different ^{[1, 2]}.

Generic sildenafil has been available since December 2017 (when the Viagra patent expired) and has dramatically reduced the cost of treating erectile dysfunction ^{[9, 11]}.

Generic sildenafil for ED typically costs $1-$5 per tablet at most pharmacies with a discount coupon (GoodRx, RxSaver, etc.), compared to $60-$80+ per tablet for brand-name Viagra ^[11].

A common cost-saving strategy: ask your doctor to prescribe 100 mg tablets and cut them in half with a pill splitter ^[11]. This effectively halves the per-dose cost, and the 100 mg tablets cost the same as lower strengths at most pharmacies.

Most insurance plans do not cover sildenafil for erectile dysfunction, or they impose significant restrictions (quantity limits, prior authorization) ^[11]. However, sildenafil for PAH (Revatio) is typically covered by insurance.

For PAH, generic sildenafil 20 mg tablets are available and significantly less expensive than brand-name Revatio ^{[2, 9]}.

Patient assistance programs: Pfizer offers assistance programs for eligible uninsured patients. Various telehealth platforms offer sildenafil at competitive prices with online consultations ^[11].

Pregnancy: Sildenafil for ED (Viagra) is not indicated for use in women ^[1]. For PAH in women of childbearing potential, sildenafil is classified as FDA Pregnancy Category B — animal studies have not shown fetal risk, but there are no adequate human studies ^[2]. PAH itself carries significant maternal mortality risk during pregnancy (estimated 30-56%) ^{[2, 5]}. The decision to continue sildenafil during pregnancy in PAH patients must weigh the life-threatening nature of PAH against potential fetal risks. Consult a high-risk obstetrician and pulmonologist.

Breastfeeding: There are no data on sildenafil excretion in human breast milk ^{[1, 2]}. For PAH patients, the decision to breastfeed while on sildenafil should consider the importance of the drug to the mother and the potential risk to the infant ^[12].

Generic sildenafil became available in December 2017 and is now manufactured by numerous companies ^[9]. It is AB-rated by the FDA as bioequivalent to Viagra.

There is no clinically meaningful difference between generic sildenafil and brand-name Viagra ^[9]. Both contain the same active ingredient, at the same strength, in the same dosage form, and must meet identical FDA quality standards.

The primary difference is price: generic sildenafil costs 90-98% less than brand-name Viagra ^[11]. Brand-name Viagra is now rarely prescribed or dispensed.

Pfizer also markets an authorized generic (identical to Viagra in all respects, including inactive ingredients) through its own subsidiary ^[9].

For PAH, generic sildenafil 20 mg tablets are available as an alternative to brand-name Revatio, also at significant cost savings ^{[2, 9]}. The original Goldstein et al. trial data ^[4] applies equally to generic formulations since bioequivalence is required.