The most common side effects of losartan are surprisingly mild and resemble cold or flu symptoms: headache, stuffy or runny nose, sore throat, body aches, and fatigue. Most people tolerate losartan well, and many of these effects ease as your body adjusts to the medication. But there are a few less common reactions worth knowing about, including effects on your potassium levels and kidney function that your doctor will monitor through routine blood work.
The Most Common Side Effects
Losartan belongs to a class of blood pressure medications that work by blocking a hormone responsible for tightening blood vessels. When those vessels relax, blood pressure drops. That shift in pressure is what causes most of the everyday side effects people notice.
The side effects reported most frequently include:
- Headache
- Stuffy or runny nose
- Sore throat and cough
- Body aches and chills
- Unusual tiredness or weakness
- Ear congestion
- Sneezing
These overlap so much with common cold symptoms that people sometimes don’t realize the medication is the cause. If you’ve recently started losartan and feel like you’re coming down with something that never quite turns into a full cold, the drug is a likely explanation. These effects are generally mild and tend to improve within the first few weeks of treatment.
Dizziness and Low Blood Pressure
Because losartan’s entire job is to lower blood pressure, dizziness is one of the most noticeable effects, especially in the first few days. You’re most likely to feel lightheaded when you stand up quickly, get out of bed, or change positions. This happens because your blood pressure drops faster than your body can compensate for, temporarily reducing blood flow to your brain.
Staying well hydrated helps. So does standing up slowly, particularly in the morning. If dizziness persists beyond the first couple of weeks or causes you to feel faint, that’s a sign your dose may need adjusting. Dehydration from illness, hot weather, or intense exercise can amplify this effect because your blood volume is already lower than usual.
Effects on Potassium Levels
Losartan can cause your body to retain slightly more potassium than normal. For most people, the increase is small, roughly 0.1 to 0.3 mmol/L, and not clinically significant. But in certain situations, potassium can climb high enough to become a concern, a condition called hyperkalemia.
You’re at higher risk if you have chronic kidney disease, heart failure, or already elevated potassium levels at baseline (above 5.0 mmol/L). Potassium-based salt substitutes are a common, overlooked contributor. Many people switch to these products to reduce sodium intake, not realizing they contain potassium chloride. The National Kidney Foundation specifically warns against using salt substitutes or seasonings containing potassium chloride while on medications like losartan.
Mild hyperkalemia often causes no symptoms at all, which is why your doctor orders periodic blood tests after starting the medication. When potassium does climb noticeably, you might feel muscle weakness, numbness, or an irregular heartbeat. High-potassium foods like bananas, oranges, and potatoes don’t need to be eliminated, but eating very large amounts of them consistently could tip the balance, particularly if your kidneys aren’t filtering at full capacity.
Kidney Function Changes
It’s normal for losartan to cause a small bump in creatinine, a waste product your kidneys filter out. Creatinine levels are a standard marker of kidney health, and increases of up to 30% within the first few weeks of treatment are considered a normal response to the drop in blood pressure, not a sign of kidney damage. Research published in the American Heart Association’s journal Hypertension confirms that increases in this range are not associated with worse kidney outcomes over time.
If creatinine rises more than 30%, that warrants investigation. It could indicate that blood flow to the kidneys has dropped too much, sometimes due to dehydration, use of anti-inflammatory painkillers like ibuprofen, or an underlying narrowing of the arteries that supply the kidneys. This is another reason blood work is typically checked a few weeks after starting or increasing the dose.
Rare but Serious Reactions
Angioedema, a sudden swelling of the face, lips, or tongue, can occur with losartan, though the risk is lower than with the closely related class of drugs called ACE inhibitors (where it affects roughly 0.1% to 0.7% of users). With losartan, the rate is thought to be even smaller. The swelling develops without itching or hives, which distinguishes it from a typical allergic reaction. In rare cases, swelling can involve the airway and become life-threatening. If you notice sudden swelling of your lips, tongue, or throat after starting losartan, seek emergency care.
Pregnancy and Fetal Risk
Losartan carries the FDA’s strongest safety warning, a boxed warning, regarding use during pregnancy. The medication can cause serious harm to a developing fetus, particularly during the second and third trimesters. It interferes with fetal kidney development, which can lead to dangerously low amniotic fluid levels and, in severe cases, fatal outcomes. The FDA’s labeling on losartan is direct: when pregnancy is detected, the drug should be stopped as soon as possible. If you’re of childbearing age and taking losartan, having a plan for contraception or a medication switch before becoming pregnant is important.
Foods and Substances That Interact
Beyond potassium-rich salt substitutes, a few other interactions are worth noting. Anti-inflammatory painkillers (ibuprofen, naproxen) can blunt losartan’s blood pressure-lowering effect and increase the risk of kidney problems when used together regularly. Occasional use for a headache is generally fine, but daily reliance on these painkillers alongside losartan creates a compounding strain on the kidneys.
Alcohol can intensify the blood pressure drop and worsen dizziness, especially in the first few weeks. This doesn’t mean you need to avoid alcohol entirely, but heavy drinking on losartan will make lightheadedness worse and can cause fainting. Lithium levels can also rise when taken alongside losartan, so anyone on both medications needs closer monitoring.
What to Expect in the First Few Weeks
Most side effects show up within the first days to weeks of starting losartan or increasing the dose. Dizziness and fatigue tend to be most noticeable early on and gradually fade as your cardiovascular system adjusts to the lower blood pressure. The cold-like symptoms (congestion, sore throat, body aches) follow a similar pattern for many people.
Your doctor will typically order blood work within two to four weeks of starting treatment to check potassium levels and kidney function. If everything looks stable at that point, monitoring moves to a less frequent schedule. If you experience persistent or worsening symptoms beyond the initial adjustment period, that’s worth reporting, as it may mean the dose is too high or a different medication would be a better fit.