Viagra Drug Interactions: What to Avoid and What to Tell Your Doctor

Sildenafil (Viagra) interacts with several medications, foods, and supplements. Some of these interactions are merely inconvenient — they may reduce Viagra's effectiveness or increase side effects. Others are potentially dangerous or even life-threatening. Here's a comprehensive guide so you know exactly what to tell your doctor and pharmacist.

Absolute Contraindication: Nitrate Medications

This is the most critical interaction. Sildenafil is absolutely contraindicated with any nitrate medication. The combination can cause a sudden, severe, potentially fatal drop in blood pressure.

Nitrate medications include:

• Nitroglycerin (Nitrostat, Nitro-Dur, Nitromist) — used for chest pain (angina)
• Isosorbide mononitrate (Imdur) and isosorbide dinitrate (Isordil)
• Amyl nitrite and amyl nitrate ("poppers" — recreational drugs)

If you take nitroglycerin, do not take Viagra. If you take Viagra and develop chest pain, do NOT take nitroglycerin — call 911 immediately. The timing of when nitrates can be safely administered after sildenafil use is unclear; some cardiologists advise waiting at least 24 hours.

Absolute Contraindication: GC Stimulators (Riociguat)

Sildenafil is also contraindicated with guanylate cyclase (GC) stimulators including:

• Riociguat (Adempas) — used for pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension
• Vericiguat (Verquvo) — used for heart failure

These drugs work on the same pathway as sildenafil and combining them causes dangerous blood pressure drops.

Major Interaction: Alpha-Blockers

Alpha-blockers lower blood pressure and are used for hypertension and enlarged prostate (BPH). Combining them with sildenafil can cause an additive blood pressure drop — sometimes resulting in dizziness, lightheadedness, or fainting. Common alpha-blockers include:

• Tamsulosin (Flomax) — for BPH
• Doxazosin (Cardura), terazosin (Hytrin), prazosin (Minipress) — for hypertension or BPH
• Alfuzosin (Uroxatral), silodosin (Rapaflo)

If you take an alpha-blocker, sildenafil can still be used, but you should be on a stable alpha-blocker dose first, and sildenafil should be started at 25 mg. Tell your doctor about all medications you take.

Major Interaction: HIV Protease Inhibitors and Strong CYP3A4 Inhibitors

Sildenafil is primarily metabolized by the liver enzyme CYP3A4. Drugs that inhibit this enzyme cause sildenafil blood levels to rise significantly — increasing both efficacy and side effects. The most extreme example is ritonavir (an HIV protease inhibitor), which increases sildenafil exposure by 11-fold. The recommended maximum dose with ritonavir is 25 mg per 48 hours.

Other strong CYP3A4 inhibitors that require a sildenafil dose reduction (start at 25 mg):

• Ketoconazole and itraconazole (antifungals)
• Clarithromycin (antibiotic)
• Erythromycin (antibiotic) — increases sildenafil levels ~3-fold
• Saquinavir, amprenavir, and other HIV protease inhibitors

Moderate Interaction: Blood Pressure Medications

Antihypertensives (amlodipine, beta-blockers, ACE inhibitors, ARBs) can cause additional blood pressure lowering when combined with sildenafil. The average additional drop with amlodipine at 100 mg sildenafil is about 8 mmHg systolic and 7 mmHg diastolic — generally manageable, but worth monitoring. Report any dizziness or lightheadedness to your doctor.

Food and Substance Interactions

Beyond prescription medications, be aware of these substance interactions:

• Grapefruit juice: Inhibits CYP3A4 in the gut, potentially increasing sildenafil blood levels and worsening side effects like low blood pressure and headache.
• Alcohol: Can worsen low blood pressure and dizziness. Sildenafil studies at 0.5 g/kg blood alcohol (~0.08%) didn't show a significant additive hypotensive effect — but higher alcohol consumption may.
• St. John's Wort: A CYP3A4 inducer — may decrease sildenafil levels and reduce its effectiveness.
• High-fat meals: Slow sildenafil absorption, delaying onset of action. Not dangerous, but worth knowing.

What to Tell Your Doctor and Pharmacist

Before starting sildenafil, give your provider and pharmacist a complete list of all medications — including OTC drugs, supplements, and vitamins. Pay special attention to:

• Any chest pain medications, especially nitroglycerin or long-acting nitrates
• Any blood pressure medications, including alpha-blockers
• Any HIV medications, particularly ritonavir-containing regimens
• Any antifungal or antibiotic medications you may be currently taking

For a full overview of Viagra side effects — both from the drug alone and from interactions — see: Viagra Side Effects: What to Expect and When to Call Your Doctor.