Olmesartan: what it treats, how to take it, side effects
Last updated September 3, 2026.
Olmesartan is an angiotensin II receptor blocker - it blocks the natural substances that tighten blood vessels, so blood flows more smoothly and the heart pumps more efficiently. You may know it as Benicar. It is the ARB with one unusual warning: a severe, chronic diarrhea that mimics celiac disease.
What does it treat?
High blood pressure in adults and children 6 and older, alone or combined with other medications. Untreated hypertension damages the brain, heart, blood vessels, and kidneys over time; the drug plus the lifestyle half - low-salt diet, healthy weight, daily exercise, no smoking, moderate alcohol - is the prevention. It controls blood pressure; it does not cure it.
How do you take it?
One tablet once a day, with or without food, same time every day. Your doctor starts low and may increase after 2 weeks - which is also about how long the full effect takes, though pressure starts dropping in the first week. Children who cannot swallow tablets can get a pharmacist-prepared liquid. Keep taking it when you feel well; never stop without your doctor. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: most people feel nothing on olmesartan - the reference page lists no routine side-effect roster. Dizziness in the first days is the one to expect as pressure adjusts.
- Serious - call your doctor immediately: swelling of the face, throat, tongue, lips, eyes, hands, feet, ankles, or lower legs; severe diarrhea; weight loss; difficulty breathing or swallowing; and hoarseness.
Who should not take it?
Anyone pregnant - olmesartan can kill or seriously injure a fetus in the last 6 months of pregnancy, so if you become pregnant, stop it and call your doctor immediately. Diabetes plus aliskiren is a do-not-combine. NSAIDs like ibuprofen and naproxen and potassium supplements are named interactions - flag them before starting. Report kidney problems and heart failure history. The diarrhea warning is specific to this ARB: severe, chronic diarrhea with weight loss months into treatment is a recognized drug effect - sprue-like enteropathy - and it resolves when the drug is switched, so never sit on it.
When is it an emergency?
Swelling of the face, tongue, or throat, or trouble breathing or swallowing is a 911 call. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why did my doctor warn me about diarrhea on a blood pressure pill?
Because olmesartan has a specific, documented effect: sprue-like enteropathy - severe, chronic diarrhea with weight loss that can appear months or even years into treatment and looks like celiac disease. It is on the MedlinePlus serious list as severe diarrhea and weight loss. The good news: it resolves after the drug is stopped and switched. The bad news: it gets missed for a long time when nobody connects it to the pill - so persistent diarrhea on olmesartan is always a call.
Why can't I take olmesartan while pregnant?
It can cause death or serious injury to the fetus when taken in the last 6 months of pregnancy - MedlinePlus's instruction is stop the drug and call your doctor immediately if you become pregnant on it. The kidney and blood-pressure system the drug blocks is one the fetus needs. Planning a pregnancy means switching regimens in advance, not after.
Why did my pharmacist ask about my potassium supplement?
Potassium supplements are a named interaction with olmesartan - ARBs already raise potassium, and adding more can push it too high. NSAIDs like ibuprofen and naproxen are on the list too, and diabetes plus aliskiren is a flat do-not-combine. Do not start any of them while on olmesartan without discussing it first.
Why does olmesartan not give me the cough my old pill did?
That old pill was probably an ACE inhibitor - their cough comes from bradykinin buildup. Olmesartan is an ARB: it blocks the same system one step later, past where bradykinin accumulates, so the cough mechanism never engages. That class difference is exactly why ARBs are the standard switch when an ACE-inhibitor cough will not quit.
Why do I have to keep taking it when I feel fine?
Because fine is the drug working. MedlinePlus: olmesartan controls high blood pressure but does not cure it, and the damage it prevents - to heart, brain, kidneys, vessels - is silent until it is done. The full effect takes about 2 weeks to arrive; stopping it takes the protection with it, with no symptom to warn you.
What happens if I miss a dose of olmesartan?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. Once-daily drugs forgive one slip. Same-time-every-day is the whole adherence strategy for a pill you cannot feel.