The most common side effect of olmesartan is headache, affecting up to 7% of people who take it. Dizziness is the second most frequent, reported by about 3% of users. Most side effects are mild or moderate and tend to occur more often at higher doses. Beyond these everyday nuisances, olmesartan carries one rare but serious risk that sets it apart from other blood pressure drugs in its class: a gut condition that can cause chronic diarrhea and significant weight loss.
Headache, Dizziness, and Other Frequent Side Effects
In placebo-controlled clinical trials, headache and dizziness consistently top the list. Dizziness shows a clear dose-dependent pattern: it affects about 2.8% of people on a low dose, 3% on 20 mg daily, and 4.1% on 40 mg daily, compared to less than 1% on placebo. In pediatric studies, headache was even more common at higher doses, reaching nearly 15% in children aged 6 to 16 on the higher dose versus about 7% on the lower one.
Other side effects reported in trials include upper respiratory symptoms (sore throat, cough, runny nose), nausea, and occasional chest discomfort. None of these occurred significantly more often than they did in people taking a sugar pill, which is why olmesartan is generally considered well tolerated. The vast majority of side effects in clinical trials were rated as mild or moderate in severity.
Why Dizziness Happens
Olmesartan lowers blood pressure by blocking a hormone called angiotensin II from tightening your blood vessels. When that signal is blocked, your arteries relax and widen, which reduces resistance and brings blood pressure down. The trade-off is that your blood pressure can sometimes drop lower than your body is used to, especially when you first start the medication or move to a higher dose. That’s what causes the lightheaded or dizzy feeling.
People who are already low on fluids or salt are most vulnerable. If you take a water pill (diuretic) alongside olmesartan, the combined effect on fluid balance can make blood pressure dip further. Standing up quickly, exercising in heat, or not drinking enough water can amplify this. Most people find the dizziness improves as their body adjusts over the first week or two.
Effects on Potassium and Kidney Markers
Because olmesartan reduces a hormone called aldosterone, which normally tells your kidneys to hold onto sodium and release potassium, the drug can cause potassium levels to creep upward. In a large health system study, mildly elevated potassium occurred in about 11% of patients on blood pressure drugs that work through this same hormonal pathway over a three-year period. Meaningfully high levels occurred in roughly 2% to 3%.
Clinical trials also noted small decreases in hemoglobin and slight increases in creatinine, a marker of kidney workload. These shifts are typically minor and don’t cause symptoms, but they’re the reason bloodwork is usually checked within the first few weeks of starting olmesartan and periodically afterward, especially if you have kidney disease or diabetes.
Sprue-Like Enteropathy: A Rare but Serious Risk
In 2013, the FDA added a warning to olmesartan’s label for a gut condition called sprue-like enteropathy. This is severe, chronic diarrhea accompanied by substantial weight loss, sometimes 40 pounds or more. Nausea, vomiting, abdominal pain, and fatigue can also be present. What makes it tricky to recognize is that it can develop months or even years after starting the medication, long after most people and their doctors have stopped thinking about new drug side effects.
The condition resembles celiac disease. Biopsies of the intestinal lining show the same kind of damage: flattened villi, the tiny finger-like projections that absorb nutrients. But unlike celiac disease, a gluten-free diet doesn’t help, and the blood tests used to diagnose celiac come back negative. The FDA identified 23 serious cases in its adverse event database, and some patients required hospitalization.
The good news is that every patient improved after stopping olmesartan. This condition has not been linked to any other drug in olmesartan’s class (angiotensin receptor blockers), making it unique to this specific medication. If you develop unexplained chronic diarrhea and weight loss while taking olmesartan, that connection is worth raising with your prescriber, even if you’ve been on the drug for years.
Drug Interactions That Can Worsen Side Effects
Certain medications amplify olmesartan’s effects on the kidneys and potassium levels. Common anti-inflammatory painkillers like ibuprofen and naproxen can reduce kidney blood flow when combined with olmesartan, raising the risk of kidney strain and pushing potassium higher. This combination is especially relevant because many people take these painkillers regularly for arthritis or back pain without thinking of them as interacting drugs.
For people on lithium, olmesartan poses a more specific risk. Lithium is cleared almost entirely by the kidneys, and olmesartan’s effect on kidney function and fluid balance can cause lithium to accumulate to dangerous levels. Studies have found dramatic increases in lithium toxicity hospitalizations within a month of starting similar blood pressure drugs in older adults. The interaction can cause neurological symptoms, confusion, and tremor.
Taking olmesartan alongside diuretics, particularly at higher diuretic doses, increases the chance of blood pressure dropping too low and of electrolyte imbalances. If you’re starting olmesartan while already on a diuretic, the initial dizziness and lightheadedness may be more pronounced than it would be otherwise.
Higher Dose, More Side Effects?
The relationship between dose and side effects is modest but real. In controlled studies comparing 5 mg, 20 mg, and 40 mg doses, dizziness climbed from 2.8% to 4.1% as the dose increased. Headache followed a similar pattern in pediatric trials, roughly doubling between the low and high doses. One analysis noted that patients on 20 mg per day had slightly more adverse events overall than those on other doses, though no single side effect stood out as dramatically worse.
For most people, the 20 mg starting dose provides a meaningful blood pressure reduction without a large jump in side effect risk. Moving to 40 mg adds a small increment of dizziness risk but is otherwise similar in its side effect profile. The decision to increase the dose typically comes down to whether blood pressure has reached its target, balanced against how the current dose feels day to day.