Sildenafil

• Erectile Dysfunction
• Pulmonary Arterial Hypertension
• Sexual Dysfunction

Sildenafil is a phosphodiesterase type 5 inhibitor with two distinct uses tied to two different brand names. As Viagra it is one of the most widely prescribed treatments for erectile dysfunction in adult men. As Revatio it is used at lower divided doses to treat pulmonary arterial hypertension. Both indications rely on the same molecular mechanism — enhanced nitric oxide signaling and vascular smooth muscle relaxation — but the two formulations are not interchangeable, and patients should be aware that the dose, schedule, and clinical context differ substantially between the two indications.

Mechanism of Action

Sildenafil reversibly and selectively inhibits phosphodiesterase type 5, the enzyme that breaks down cyclic guanosine monophosphate (cGMP) in vascular smooth muscle. Nitric oxide, released by endothelial cells in response to local stimuli, activates guanylate cyclase to produce cGMP. Rising intracellular cGMP relaxes smooth muscle by lowering calcium availability, producing vasodilation. By inhibiting the enzyme that degrades cGMP, sildenafil amplifies and prolongs the vasodilatory response to whatever nitric oxide is being released — it does not generate erections or vasodilation on its own.

In the corpus cavernosum, sexual stimulation triggers neuronal nitric oxide release, which initiates the cGMP cascade. With sildenafil on board, that cascade leads to fuller and more sustained smooth muscle relaxation, increased arterial inflow, and the venous compression that maintains penile rigidity. Because the drug requires sexual stimulation to be effective, it does not produce spontaneous erections. In the pulmonary vasculature, where PDE5 is also abundantly expressed, the same mechanism reduces pulmonary vascular resistance, lowers mean pulmonary artery pressure, and improves right ventricular function and exercise capacity in patients with pulmonary arterial hypertension. The drug has minor activity at PDE6 in the retina, which produces the transient blue-tinge or light sensitivity some patients notice, and at PDE1 in the heart and arteries, generally without clinical importance. Sildenafil is metabolized primarily by CYP3A4, with secondary involvement of CYP2C9, which underlies the dominant drug interactions. Onset of effect occurs within 30 to 60 minutes, and duration is roughly four to six hours for the ED indication. Background on erectile dysfunction from the Urology Care Foundation is useful patient reading.

Clinical Use

For erectile dysfunction the American Urological Association positions PDE5 inhibitors as first-line pharmacologic therapy after addressing reversible contributors — cardiovascular disease, diabetes, depression, hypogonadism, sleep disturbance, and certain medications. Sildenafil competes with tadalafil and other PDE5 inhibitors. Tadalafil offers a longer duration of action (up to 36 hours) and a daily low-dose option that supports spontaneity, while sildenafil is taken on demand and acts for roughly four to six hours. Choice often comes down to patient preference, response, cost, and partner factors. In comparative effectiveness studies, all PDE5 inhibitors are similarly effective overall, with about 60 to 80 percent of men achieving satisfactory erections.

For pulmonary arterial hypertension (WHO Group 1), sildenafil is one of several pulmonary vasodilators alongside endothelin receptor antagonists and prostacyclin pathway agents. It is generally used in mild-to-moderate functional class II and III patients, often as part of combination therapy. Patient selection for the ED indication is largely about cardiovascular safety: the activity required for sexual intercourse should be tolerable from a cardiac standpoint, and the patient must not be on nitrates or riociguat. Cardiac risk stratification before prescribing follows the Princeton Consensus framework. Reading our men's health checklist helps frame the broader workup that often accompanies ED evaluation. ED is increasingly recognized as a sentinel marker of vascular disease that may precede coronary events by three to five years, which makes a thorough cardiovascular history and risk-factor assessment as important as the prescription itself. Patients with poor response should be reassessed for adequate sexual stimulation, correct timing relative to meals, and dose optimization before declaring treatment failure.

How to Take It

For erectile dysfunction the typical starting dose is 50 mg taken roughly one hour before sexual activity, with a dose range of 25 to 100 mg and a maximum of one dose per day. Onset is generally 30 to 60 minutes; effect lasts four to six hours. Absorption is slowed by a high-fat meal, which can delay onset by an hour or more — a lighter meal or an empty stomach is preferred when timing matters. Alcohol does not reduce efficacy directly but can blunt sexual response and worsen orthostatic effects. For pulmonary arterial hypertension the dose is 20 mg three times daily, four to six hours apart, taken consistently with or without food. Missed PAH doses should be taken when remembered if not close to the next dose; double dosing is not recommended. Tablets are stored at room temperature in original packaging. The first dose may produce facial flushing, mild headache, nasal congestion, and a slight blue tinge to vision; these typically diminish with subsequent doses. Patients should not combine sildenafil with grapefruit juice in large quantities because of CYP3A4 inhibition that raises plasma levels.

Monitoring and Follow-Up

For the ED indication, monitoring is mostly clinical: response to therapy, side effect tolerability, and ongoing cardiovascular risk assessment. Baseline blood pressure, a focused cardiac history, and review of all medications including over-the-counter nitrates and "poppers" are essential. Lipid panels, fasting glucose or A1c, and testosterone are often obtained as part of an ED workup because erectile dysfunction is frequently a sentinel finding for vascular and metabolic disease. Follow-up at four to eight weeks reviews efficacy, side effects, and any cardiac symptoms experienced during sexual activity. For pulmonary arterial hypertension, monitoring is much more intensive: six-minute walk test distance, NT-proBNP, echocardiography, and right heart catheterization at intervals defined by the pulmonary hypertension specialist guide therapy adjustments. Vision changes, sudden hearing loss, or any erection lasting more than four hours are red events requiring immediate care. Blood pressure trends should be reviewed if patients are on multiple antihypertensives, and orthostatic measurements help detect early problems. PAH patients are followed every three to six months at minimum because the disease itself can progress despite stable therapy. Background information from the NHLBI on pulmonary hypertension supports patient education.

Special Populations

In older adults a 25 mg starting dose for ED is prudent because of slower clearance and higher susceptibility to orthostatic effects. Hepatic impairment reduces clearance; ED dosing typically starts at 25 mg, with no specific adjustment for the PAH indication although caution applies. Severe renal impairment likewise warrants a 25 mg start for ED. Sildenafil is not indicated for women with sexual dysfunction. The PAH formulation is approved for adults; pediatric use for PAH is generally avoided after a trial showed increased mortality at higher doses. Patients on strong CYP3A4 inhibitors such as ritonavir, ketoconazole, or itraconazole should not exceed 25 mg in a 48-hour period for ED because plasma levels can rise sharply. Concurrent nitrates of any kind — daily, sublingual, or recreational amyl nitrite — are an absolute contraindication because of life-threatening hypotension. Riociguat is similarly contraindicated. Alpha-blockers such as tamsulosin are usable but require stable dosing of the alpha-blocker first to minimize additive hypotension. Patients with hereditary degenerative retinal disorders such as retinitis pigmentosa should be counseled about theoretical concerns. Men with sickle cell disease or anatomic predisposition to priapism need careful counseling about that risk.

When to Contact Your Doctor

Call immediately for chest pain during or after sexual activity, sudden vision loss in one or both eyes, or sudden hearing loss with or without ringing — these may signal serious cardiovascular or sensory complications. An erection lasting more than four hours (priapism) is a urologic emergency requiring same-day care to prevent permanent damage. Severe dizziness or fainting, especially when combined with other blood pressure medications, deserves prompt evaluation. New shortness of breath, ankle swelling, or fatigue in patients on the PAH formulation may indicate worsening pulmonary hypertension. Skin reactions, severe headaches, or allergic symptoms also warrant a call. Painful, bloody, or particularly weak ejaculation deserves a conversation. Any chest discomfort or palpitations during exertion warrant cardiology input.

For evaluation of erectile dysfunction or coordinated care of pulmonary hypertension and underlying cardiovascular risk, contact us or schedule a visit with our St. Petersburg internal medicine team for a personalized assessment.