Losartan is a blood pressure medication with three FDA-approved uses: treating high blood pressure, reducing stroke risk in people whose heart has thickened from hypertension, and protecting the kidneys in people with type 2 diabetes. It belongs to a class of drugs called angiotensin II receptor blockers, or ARBs, and is one of the most commonly prescribed medications in the United States. Most people take it as a once-daily tablet starting at 50 mg.
High Blood Pressure
The primary reason losartan is prescribed is to lower blood pressure. It’s approved for adults and children over age 6, and it’s considered a first-line treatment for stage 1 hypertension, meaning it’s one of the first medications a doctor will reach for rather than a backup option. For people without other complicating conditions like diabetes or heart failure, losartan can work well on its own. When someone has a higher risk of cardiovascular disease (10% or greater), it’s often combined with another blood pressure medication for tighter control.
How Losartan Lowers Blood Pressure
Your body has a hormone called angiotensin II that tightens blood vessels. When this hormone locks onto receptors on blood vessel walls, it triggers a chain of signals that cause the muscle around those vessels to contract, narrowing them and driving blood pressure up. Losartan blocks those receptors so angiotensin II can’t bind to them. With the signal blocked, blood vessels relax and widen, and pressure drops.
This is different from an older class of drugs called ACE inhibitors, which work one step earlier in the same system by preventing angiotensin II from being produced in the first place. ACE inhibitors are known for causing a persistent dry cough in some people because they also cause a substance called bradykinin to build up in the lungs. Losartan doesn’t have that effect, which is why it’s often prescribed as an alternative for people who developed a cough on an ACE inhibitor.
Stroke Prevention
Losartan is also approved to reduce stroke risk in people who have both high blood pressure and left ventricular hypertrophy, a condition where the heart’s main pumping chamber has grown thicker than normal from working against elevated pressure over time. This thickening is a significant risk factor for stroke and other cardiovascular events.
The evidence here is striking. In a large clinical trial called the LIFE study, losartan reduced the risk of any stroke by 40% compared to a different blood pressure medication (atenolol, a beta-blocker), even though both drugs lowered blood pressure by similar amounts. Fatal strokes dropped by 70%, and strokes caused by blood clots forming in brain arteries fell by 45%. These benefits appear to go beyond simple blood pressure control, suggesting losartan has protective effects on blood vessels themselves.
One important caveat: the clinical evidence showed this stroke-reduction benefit did not apply to Black patients. The FDA label specifically notes this, and it’s something prescribers take into account when choosing between blood pressure medications.
Kidney Protection in Type 2 Diabetes
The third approved use is treating diabetic nephropathy, which is kidney damage caused by type 2 diabetes. This indication applies specifically to people who already have both elevated creatinine (a waste product that rises when kidneys aren’t filtering well) and significant protein leaking into their urine, along with a history of high blood pressure.
A landmark trial published in the New England Journal of Medicine followed over 1,500 patients with type 2 diabetes and kidney disease for an average of 3.4 years. Losartan reduced the risk of kidneys deteriorating to the point of needing dialysis by 28%. The risk of kidney function dropping by half was reduced by 25%. Protein in the urine, a key marker of kidney damage, fell by 35%. Hospitalizations for heart failure also dropped by 32%, an unexpected bonus. Most patients in the trial took 100 mg daily.
ARBs like losartan are considered protective for the kidneys in type 2 diabetes because blocking angiotensin II reduces pressure inside the tiny blood vessels of the kidneys, slowing the damage that high blood sugar and high blood pressure cause over time.
Off-Label Use in Marfan Syndrome
Beyond its three approved uses, losartan is sometimes prescribed off-label for Marfan syndrome, a genetic connective tissue disorder that can cause dangerous widening of the aorta. A European clinical trial called COMPARE found that losartan slowed the rate of aortic root dilation in adults with Marfan syndrome, and follow-up data over a median of 8 years suggested meaningful clinical benefit, particularly when losartan was combined with beta-blockers. While not yet an FDA-approved indication, losartan is increasingly considered a treatment option for Marfan patients, especially those already on beta-blockers who need additional aortic protection.
What to Expect When Taking It
The typical starting dose is 50 mg once a day for all three approved uses. Your prescriber may increase the dose based on how your blood pressure responds. For stroke prevention, a second medication (usually a water pill called hydrochlorothiazide) is often added alongside losartan.
Because losartan affects the same hormone system that regulates potassium levels, it can cause potassium to build up in the blood. This is especially relevant if you’re also taking potassium supplements, potassium-sparing diuretics, or certain other medications. Your doctor will typically check your kidney function and electrolyte levels two to four weeks after starting losartan, and potassium should be monitored periodically throughout treatment. Other common side effects tend to be mild: dizziness, especially in the first few days, and occasionally upper respiratory symptoms or back pain.
Pregnancy and Key Safety Concerns
Losartan carries the FDA’s strongest safety warning, a black box warning, about use during pregnancy. Drugs that act on the renin-angiotensin system can cause serious harm to a developing fetus, including kidney failure, low amniotic fluid, skeletal deformations, and death. This risk is highest during the second and third trimesters. If you become pregnant while taking losartan, it should be stopped as soon as possible.
People with severely impaired kidney function or those taking certain combinations of medications that all target the same blood pressure system need careful evaluation before starting losartan. If you’re already on an ACE inhibitor, you generally should not take losartan at the same time, as doubling up on drugs that block the same pathway increases the risk of dangerously low blood pressure, high potassium, and kidney problems.