Irbesartan 150 mg is a blood pressure medication, and most people tolerate it well. The most frequently reported side effects in clinical trials were fatigue (4% of patients), diarrhea (3%), dizziness, and heartburn (2%). These are generally mild and tend to improve within the first week or two as your body adjusts to the medication.
How Irbesartan Works and Why Side Effects Happen
Irbesartan lowers blood pressure by blocking a receptor that normally causes blood vessels to tighten and signals your body to retain salt and water. When that receptor is blocked, your blood vessels relax and your body holds onto less fluid. This is exactly what you want for blood pressure control, but it also explains the most common side effects: your body is adapting to lower pressure in your circulatory system, which can temporarily cause dizziness, lightheadedness, and fatigue.
Common Side Effects
In placebo-controlled trials involving nearly 2,000 patients taking irbesartan for high blood pressure, three side effects showed up noticeably more often than in patients taking a sugar pill:
- Fatigue: 4% of patients (vs. 3% on placebo)
- Diarrhea: 3% (vs. 2% on placebo)
- Heartburn or indigestion: 2% (vs. 1% on placebo)
Dizziness is also common, particularly when standing up quickly from a sitting or lying position. In a larger trial of patients with kidney disease and diabetes, dizziness affected about 10% of people on irbesartan compared to 6% on placebo, and lightheadedness on standing occurred in roughly 5%. Headaches have also been reported but typically resolve within the first week.
These side effects are most noticeable in the first few days of treatment and often fade as your body adjusts. If dizziness when standing is bothersome, getting up slowly and pausing for a moment before walking can help.
Potassium Levels and Kidney Function
Because irbesartan changes how your kidneys handle salt and water, it can also cause your body to retain slightly more potassium than usual. For most people, the increase is small (about 0.1 to 0.3 mmol/L) and not clinically meaningful. But if you already have kidney disease, heart failure, or take other medications that raise potassium, the effect can stack up and become a concern. Symptoms of high potassium include muscle weakness, irregular heartbeat, and tingling or numbness.
A small rise in creatinine (a marker of kidney function) is also expected when starting irbesartan. Guidelines consider an increase of up to 30% acceptable, especially in the first few weeks. Your doctor will typically check bloodwork shortly after you start the medication and periodically afterward to make sure both potassium and kidney function stay in a safe range.
Who Is More Likely to Experience Side Effects
People who are dehydrated or on high doses of water pills (diuretics) are at greater risk of a significant blood pressure drop when they first start irbesartan. This can cause pronounced dizziness, faintness, or cold sweats. If you fall into this category, your doctor may adjust your fluid intake or diuretic dose before starting irbesartan.
In clinical studies, older adults (65 and over) did not experience side effects at significantly higher rates than younger patients. That said, older individuals may be more sensitive to blood pressure changes simply because of age-related shifts in how the body regulates circulation.
Drug Interactions That Increase Risk
Certain medications can amplify irbesartan’s side effects or create new ones:
- NSAIDs (ibuprofen, naproxen, celecoxib): These common pain relievers can reduce irbesartan’s effectiveness and strain the kidneys, especially in older adults or anyone already dealing with reduced kidney function. The combination can, in some cases, lead to acute kidney problems, though these are usually reversible.
- Potassium supplements and salt substitutes: Many salt substitutes swap sodium for potassium. Combined with irbesartan’s own potassium-raising effect, this can push levels too high.
- Lithium: Irbesartan can increase lithium concentrations in the blood, raising the risk of lithium toxicity. If you take both, your lithium levels will need closer monitoring.
Serious but Rare Risks
Severe allergic reactions, including swelling of the face, lips, tongue, or throat (angioedema), have been reported with irbesartan, though they are uncommon. This type of reaction requires immediate medical attention because it can obstruct breathing.
Irbesartan carries an FDA black box warning, the most serious category, regarding pregnancy. The medication can cause severe harm to a developing baby, particularly during the second and third trimesters, including kidney failure, low amniotic fluid, and skeletal abnormalities. If you become pregnant while taking irbesartan, it should be stopped as soon as possible.
Past Recall Due to Impurity
Some batches of generic irbesartan were voluntarily recalled after trace amounts of NDEA, a substance classified as a probable carcinogen, were detected during manufacturing quality checks. No adverse events were reported in connection with the affected batches. If you’re concerned about whether your current prescription was part of a recall, your pharmacist can verify the lot number. The important thing is not to stop taking the medication abruptly without a replacement, since the risk of uncontrolled blood pressure is more immediate than the trace-level impurity.