The Complete Viagra (Sildenafil) Buying Guide

What Is Viagra?

Viagra is the brand name for sildenafil, a medication belonging to the phosphodiesterase type 5 (PDE5) inhibitor class. It was the first oral medication approved for the treatment of erectile dysfunction (ED) and has become one of the most widely recognized prescription medications in the world.

Sildenafil was originally developed by Pfizer as a treatment for pulmonary arterial hypertension (PAH) and high blood pressure. During clinical trials, researchers observed its effect on penile erections, leading to its approval for ED in 1998.

Unlike opioid medications, sildenafil is not a controlled substance. It is a prescription medication that does not carry abuse potential or risk of dependence. Generic sildenafil has been available since 2017, making it more affordable and accessible.

How Viagra Works

Sildenafil works by inhibiting the enzyme phosphodiesterase type 5 (PDE5). PDE5 is primarily found in the smooth muscle of blood vessels in the penis (corpus cavernosum) and the lungs.

During sexual arousal, the nerves and endothelial cells in the penis release nitric oxide (NO). Nitric oxide activates the enzyme guanylate cyclase, which increases levels of cyclic guanosine monophosphate (cGMP). cGMP causes smooth muscle relaxation and increased blood flow into the penis, producing an erection.

PDE5 normally breaks down cGMP, terminating the erection. By inhibiting PDE5, sildenafil allows cGMP to accumulate and persist, enhancing and prolonging the erection. Importantly, sildenafil does not cause an erection on its own — sexual stimulation is still required.

Sildenafil also has some effect on PDE6 in the retina, which can cause visual side effects. It has a half-life of approximately 3 to 5 hours and reaches peak blood levels in 30 to 120 minutes.

Classification & Legal Status

FDA-Approved Uses

Erectile Dysfunction (ED)

Sildenafil is FDA-approved for the treatment of erectile dysfunction (ED), defined as the inability to achieve or maintain a penile erection sufficient for satisfactory sexual performance. ED affects an estimated 30 million men in the United States and becomes more common with age.

Pulmonary Arterial Hypertension (PAH)

When marketed as Revatio, sildenafil is FDA-approved for the treatment of pulmonary arterial hypertension (WHO Group 1) to improve exercise capacity. The dose for PAH (20 mg three times daily) is different from the dose used for ED.

Revatio is a different formulation and indication from Viagra and should not be used interchangeably.

Dosage Forms & Strengths

Oral Tablets (Viagra)

The usual starting dose is 50 mg taken approximately 30 to 60 minutes before sexual activity. The dose may be increased to 100 mg or decreased to 25 mg based on efficacy and tolerability. The maximum recommended dose is 100 mg, and it should not be taken more than once per day.

Revatio (for PAH)

Elderly patients (=65 years) and those with hepatic impairment should start at 25 mg. Patients taking CYP3A4 inhibitors (ritonavir, ketoconazole) should start at 25 mg.

Side Effects

Common Side Effects

• Headache — the most common side effect (up to 16% of users)
• Facial flushing — redness and warmth of the face
• Nasal congestion — stuffy or runny nose
• Dyspepsia (indigestion) — stomach discomfort
• Dizziness
• Visual changes — blue-tinted vision, increased sensitivity to light (due to PDE6 inhibition in the retina)
• Muscle pain — back pain, limb pain

Serious Side Effects

Contact your healthcare provider or seek emergency medical attention if you experience:

• Priapism — a painful erection lasting more than 4 hours. This is a medical emergency that can cause permanent penile damage if not treated promptly.
• Severe hypotension — dangerously low blood pressure, especially when combined with nitrates
• Sudden vision loss — non-arteritic anterior ischemic optic neuropathy (NAION); rare but serious
• Sudden hearing loss — sudden decrease or loss of hearing, sometimes with tinnitus and dizziness
• Severe allergic reaction (anaphylaxis)
• Severe skin reactions — Stevens-Johnson syndrome (very rare)

Drug Interactions

Major Interactions (CONTRAINDICATED)

• Nitrates (nitroglycerin, isosorbide dinitrate, isosorbide mononitrate, amyl nitrite, nitroprusside) — CONTRAINDICATED. The combination causes severe, potentially life-threatening hypotension. This is the most important drug interaction to know.
• Nitric oxide donors (molsidomine) — same mechanism as nitrates
• Recreational nitrates ("poppers" — amyl nitrite, butyl nitrite) — same dangerous interaction

Major Interactions (Use with Extreme Caution)

• Alpha-blockers (doxazosin, tamsulosin, prazosin) — additive hypotension; start sildenafil at lowest dose if using alpha-blockers
• CYP3A4 inhibitors (ritonavir, ketoconazole, erythromycin, itraconazole) — dramatically increase sildenafil levels; dose reduction required
• Other PDE5 inhibitors (tadalafil, vardenafil) — do not combine

Moderate Interactions (Monitor Closely)

• Amlodipine and other calcium channel blockers — additive blood pressure lowering
• CYP3A4 inducers (rifampin, phenytoin, carbamazepine) — may decrease sildenafil effectiveness
• Alcohol — may increase risk of hypotension; limit intake

Precautions & Warnings

• Nitrate Contraindication: Sildenafil is absolutely contraindicated with any form of nitrates. Always inform your healthcare provider if you take nitrates for chest pain (angina).
• Cardiovascular disease: Sexual activity carries cardiovascular risk. Evaluate cardiac status before prescribing sildenafil for ED.
• Priapism risk: Patients with conditions predisposing to priapism (sickle cell anemia, multiple myeloma, penile deformities) should use caution.
• Vision and hearing loss: Rare cases of sudden vision loss (NAION) and sudden hearing loss have been reported. Patients should stop sildenafil and seek immediate medical attention if these occur.
• Pregnancy and breastfeeding: Viagra is intended for use in men. Sildenafil has been used off-label in pregnancy for fetal growth restriction but should only be used under specialist guidance.
• Liver disease: Metabolized by the liver. Dose reduction required in hepatic impairment.
• Kidney disease: Dose adjustment may be needed in severe renal impairment.
• Driving: Sildenafil may cause dizziness. Use caution.

When to Contact a Clinician

• Sildenafil is not working effectively for your erectile dysfunction
• You experience side effects that are bothersome or persistent
• You experience a painful erection lasting more than 4 hours (priapism) — seek emergency care
• You experience sudden changes in vision or hearing — seek emergency care
• You are starting or stopping any other medication, especially nitrates or alpha-blockers
• You have new onset chest pain or cardiac symptoms
• You are considering using sildenafil for a purpose other than ED or PAH

Seek emergency care immediately if you experience priapism (erection lasting >4 hours), sudden vision loss, sudden hearing loss, severe hypotension, chest pain, or severe allergic reaction.