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

Isosorbide mononitrate and isosorbide dinitrate interact with a wide range of medications, supplements, and substances. Most interactions involve blood pressure — isosorbide lowers BP, and anything else that does the same can cause additive drops that lead to dizziness, fainting, or dangerous cardiovascular collapse.

This guide covers the most important interactions — ranked from most to least dangerous — so you know exactly what to discuss with your doctor and pharmacist.

The Most Dangerous Interaction: Erectile Dysfunction Medications

This combination is absolutely contraindicated and can be fatal. Combining isosorbide with any PDE5 inhibitor — including:

• Sildenafil (Viagra, Revatio)
• Tadalafil (Cialis, Adcirca)
• Vardenafil (Levitra)
• Avanafil (Stendra)

...causes a synergistic, potentially fatal drop in blood pressure. Both drugs dilate blood vessels through related pathways (isosorbide via nitric oxide; PDE5 inhibitors by blocking cGMP breakdown). Together, vasodilation becomes extreme. Clinical studies showed standing mean maximum reductions in systolic/diastolic BP of -52/-29 mmHg — compared to -25/-15 mmHg with nitrates alone.

Patients must wait at least 24 hours after taking sildenafil or vardenafil before using any nitrate. For tadalafil (which stays in the body longer), the wait is 48 hours.

Second Major Contraindication: Riociguat (Adempas)

Riociguat is a soluble guanylate cyclase (sGC) stimulator used to treat pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension. Like PDE5 inhibitors, it works in the same nitric oxide/cGMP pathway as isosorbide. Combining the two causes dangerous hypotension and is absolutely contraindicated.

Major Interaction: Alcohol

Alcohol is a vasodilator. Combining alcohol with isosorbide can cause additive blood pressure lowering, enhanced dizziness, flushing, and an increased risk of fainting. If you drink alcohol, discuss safe limits with your doctor. Moderate consumption (1 drink or fewer) may be acceptable, but heavy drinking while on isosorbide is dangerous.

Moderate Interactions: Blood Pressure Medications

Many patients with coronary artery disease take multiple blood pressure medications alongside isosorbide. These combinations are usually manageable but require monitoring:

• Calcium channel blockers (amlodipine, diltiazem, verapamil): Can cause marked symptomatic orthostatic hypotension when combined with isosorbide. Dose adjustments of either agent may be needed. Monitor BP carefully when starting or adjusting doses.
• ACE inhibitors (lisinopril, enalapril) and ARBs (losartan, valsartan): Additive blood pressure lowering. Usually well-tolerated in combination, but watch for dizziness on standing.
• Beta-blockers (metoprolol, atenolol, carvedilol): Commonly co-prescribed with isosorbide. Generally safe together; beta-blockers can actually counteract reflex tachycardia from isosorbide. Monitor BP.
• Alpha-blockers (tamsulosin/Flomax, doxazosin/Cardura): Used for BPH or high blood pressure. Can cause additive hypotension with isosorbide. Monitor carefully, especially in elderly patients.

Moderate Interaction: Ergotamine / Dihydroergotamine

Ergot alkaloids (used for migraines) can counteract the vasodilating effects of isosorbide and may trigger coronary vasoconstriction. This interaction reduces isosorbide's antianginal efficacy. Tell your doctor if you take any ergotamine-containing migraine medications.

Lab Test Interference

Nitrates (including isosorbide) can interfere with the Zlatkis-Zak color reaction used in some serum cholesterol tests, producing falsely low readings. If you are getting a cholesterol panel, inform your lab or ordering provider that you are on isosorbide.

What to Tell Your Doctor and Pharmacist

Before starting isosorbide, give your healthcare team a complete list of:

• All prescription medications (especially blood pressure drugs, ED medications, and pulmonary hypertension drugs)
• Over-the-counter medications (especially decongestants and cold medicines that can raise BP)
• Herbal supplements (some, like ginkgo and hawthorn, have cardiovascular effects)
• Your typical alcohol consumption
• Any recreational drug use (particularly poppers/amyl nitrite, which are nitrates and absolutely contraindicated with isosorbide)

Key Takeaways on Isosorbide Interactions

• Never combine isosorbide with ED drugs (sildenafil, tadalafil, vardenafil, avanafil) or riociguat
• Avoid heavy alcohol use while taking isosorbide
• Other blood pressure medications can be used alongside isosorbide but need monitoring
• Tell all your providers — including dentists and urgent care doctors — that you take isosorbide

For a full review of isosorbide side effects (beyond interactions), see our guide to isosorbide side effects. If you're struggling to find your isosorbide prescription, medfinder can locate pharmacies near you with your specific formulation in stock.