Losartan is a blood pressure medication that most people tolerate well, but it does cause side effects in a meaningful number of users. The most common is dizziness, which affects somewhere between 1 and 10 out of every 100 people taking it. Most side effects are mild and tend to fade as your body adjusts, but a few deserve closer attention.
Common Side Effects
Dizziness is the side effect you’re most likely to notice. It’s reported at a “common” frequency across every group studied, whether you’re taking losartan for high blood pressure, heart failure, or kidney protection in diabetes. It tends to be most noticeable when you first start the medication or after a dose increase.
Fatigue is another frequently reported effect, though how often it occurs depends on your underlying condition. For people taking losartan strictly for high blood pressure, fatigue is uncommon (fewer than 1 in 100). But for people with thickened heart muscle from long-standing hypertension or those with type 2 diabetes and kidney disease, fatigue moves into the “common” category, affecting up to 1 in 10.
Other side effects that fall in the common-to-uncommon range include nasal congestion, back pain, and stomach discomfort. These typically don’t require medical attention and often improve within the first few weeks of treatment as your body adjusts to the medication.
Blood Pressure Dropping Too Low
Because losartan’s entire job is lowering blood pressure, it can occasionally overshoot. This is called hypotension, and it’s most likely after your very first dose. In one study of hospitalized heart patients, 24% experienced a drop in blood pressure after their first losartan dose, though this was significantly lower than the 38% rate seen with an older class of blood pressure drug (ACE inhibitors). The blood pressure drop tends to peak about 3.5 hours after taking the pill.
You’re at higher risk for this if you’re dehydrated, on a low-salt diet, taking diuretics (water pills), or have heart failure. Symptoms feel like what you’d expect from low blood pressure: lightheadedness, feeling faint when standing up quickly, or blurred vision. Staying well-hydrated and standing up slowly, especially during the first few days, helps reduce the risk.
Elevated Potassium Levels
Losartan can raise potassium levels in your blood, a condition called hyperkalemia. Your kidneys normally flush out excess potassium, but losartan interferes with a hormone system that helps regulate this process. In a clinical trial of people with type 2 diabetes and kidney disease, 7% of those on losartan developed elevated potassium compared to 3% on placebo. That’s more than double the background rate.
Mild elevations often cause no symptoms at all. Higher levels can cause muscle weakness, numbness, tingling, nausea, or an irregular heartbeat. The risk is greater if you already have kidney problems, take potassium supplements, or use salt substitutes (which are typically potassium chloride instead of sodium chloride). Your doctor will likely check your potassium through routine blood work after starting the medication.
Effects on Kidney Function
Losartan and similar medications can cause a small, expected rise in creatinine, a waste product your kidneys filter from your blood. A bump of up to 30% above your baseline is generally considered safe and may actually signal that the drug is protecting your kidneys long-term. Increases beyond 30%, however, raise concern about the balance of risks and benefits and typically prompt a conversation about whether to continue the medication.
This is why kidney function is monitored with blood tests before and after starting losartan. If you already have reduced kidney function, these checks are especially important in the first few weeks.
Angioedema: Rare but Serious
Angioedema is a sudden swelling of the deeper layers of skin, usually around the face, lips, tongue, or throat. It’s rare with losartan. A meta-analysis of randomized trials found the incidence for drugs in losartan’s class (ARBs) was about 0.11%, compared to 0.30% for ACE inhibitors. One study did suggest losartan may carry a slightly higher risk of angioedema than other ARBs, at roughly 2.3 cases per 1,000 person-years of use.
If you’ve previously had angioedema from an ACE inhibitor (drugs ending in “-pril”), your risk with losartan is lower but not zero. Swelling of the tongue or throat is a medical emergency regardless of the cause.
Pregnancy and Fetal Risk
Losartan carries the FDA’s most serious safety label, a boxed warning, for fetal toxicity. The drug can cause injury and death to a developing fetus, particularly during the second and third trimesters. It reduces fetal kidney function and can lead to dangerously low amniotic fluid, which in turn causes problems with lung and skeletal development. If you become pregnant while taking losartan, the medication should be stopped as soon as the pregnancy is confirmed.
This warning applies to the entire class of drugs that work on the same hormone system, not just losartan specifically. If you’re planning a pregnancy or could become pregnant, a different type of blood pressure medication is a safer choice.
Past Manufacturing Recalls
Between 2018 and 2019, several losartan manufacturers issued recalls after batches were found to contain nitrosamine impurities, chemicals that can increase cancer risk with long-term exposure. The contamination stemmed from specific overseas manufacturing facilities and affected certain generic versions. The FDA issued warning letters to manufacturers including Torrent Pharmaceuticals and Mylan Pharmaceuticals, and multiple lots were pulled from the market. These recalls were resolved, and the FDA implemented tighter testing standards for the drug’s active ingredient. If you were taking losartan during that period and are concerned, the exposure levels in most affected batches were low enough that the additional cancer risk was considered very small.
What the Adjustment Period Looks Like
Many of losartan’s milder side effects, particularly dizziness and fatigue, tend to improve as your body adjusts to the medication. The Mayo Clinic notes that some side effects “may go away during treatment” without any intervention. There’s no precise timeline that applies to everyone, but most people find the first one to two weeks the roughest. If dizziness or fatigue persists well beyond that window, it may be worth discussing a dose adjustment.
Taking losartan at bedtime rather than in the morning is a common strategy for managing dizziness, since the peak blood pressure drop happens while you’re asleep. Consistent hydration also makes a noticeable difference, especially in warmer weather or if you exercise regularly.