Losartan side effects: what to expect

Losartan is one of the best tolerated blood pressure medicines available. Compared to many other antihypertensives, the incidence of side effects is relatively low.

Common side effects (affecting up to 1 in 10 people):

• Dizziness or light-headedness: particularly when you first start treatment or when the dose is increased, due to the blood pressure being lowered
• Hyperkalaemia (high potassium): losartan can raise potassium levels in the blood, which is why your doctor will check blood tests when you start treatment and periodically thereafter
• Fatigue: some patients report feeling tired, particularly in the first few weeks

Uncommon side effects (affecting up to 1 in 100 people):

• Headache
• Sleep disturbances
• Palpitations
• Abdominal pain, nausea or diarrhoea
• Muscle cramps
• Mild skin rash

According to the BNF, losartan is generally better tolerated than ACE inhibitors because it does not affect bradykinin metabolism.

This means it is much less likely to cause the persistent dry cough that leads many patients to switch from medicines such as ramipril or lisinopril.

Hyperkalaemia (a potassium level above 5.5 mmol/L) is one of the most clinically significant effects of losartan and all ARBs. Understanding this risk is important for your safety.

Why losartan raises potassium:

Losartan blocks the action of angiotensin II, which normally stimulates aldosterone release. Aldosterone causes the kidneys to excrete potassium.

When aldosterone is reduced, the kidneys retain more potassium.

Who is at greatest risk?

• Patients with chronic kidney disease
• Those taking potassium supplements or potassium-sparing diuretics (such as spironolactone or amiloride)
• Patients also taking an ACE inhibitor (dual RAAS blockade)
• Elderly patients, who may have reduced kidney function
• Patients with diabetes

How it is monitored:

NICE and the BNF recommend checking renal function and electrolytes (including potassium):

• Before starting losartan
• 1 to 2 weeks after starting or changing the dose
• Periodically during long-term treatment (at least annually, or more often in patients at higher risk)

Mild hyperkalaemia may be managed by reducing potassium-rich foods or stopping interacting medicines. Severe hyperkalaemia is a medical emergency requiring hospital treatment.

Serious side effects of losartan are uncommon, but awareness is important.

Angioedema:

Swelling of the face, lips, tongue or throat can occur with any ARB, though it is rarer than with ACE inhibitors. If you develop facial swelling or difficulty breathing, call 999 immediately.

Patients who have had angioedema with an ACE inhibitor should use ARBs with caution.

Acute kidney injury:

Losartan can temporarily reduce kidney function, particularly in patients who are dehydrated, have heart failure, renal artery stenosis or are taking NSAIDs.

Signs include a significant reduction in urine output or swelling of the legs. Blood tests will detect changes in kidney function early.

Severe hypotension:

A significant drop in blood pressure can occur, particularly after the first dose, in patients who are volume-depleted (for example, those taking high-dose diuretics).

Symptoms include dizziness, fainting and blurred vision.

Liver problems:

Rarely, losartan may cause abnormal liver function tests. The BNF advises monitoring if symptoms such as unexplained nausea, dark urine or jaundice develop.

Harm in pregnancy:

All ARBs, including losartan, must not be used during pregnancy as they can cause serious harm to the unborn baby, particularly in the second and third trimesters.

Effective contraception should be used, and the medicine must be stopped as soon as pregnancy is confirmed.

One of the main reasons doctors prescribe losartan is that it does not usually cause a dry cough, which is a common and often troublesome side effect of ACE inhibitors such as ramipril, lisinopril and enalapril.

Why ACE inhibitors cause a cough:

ACE inhibitors block the breakdown of bradykinin, a substance that accumulates in the lungs and can trigger a persistent, dry, tickly cough.

This affects approximately 10 to 15% of patients and is more common in women.

Why losartan is different:

Losartan works by blocking the angiotensin II receptor directly, rather than inhibiting ACE. This means it does not affect bradykinin levels and is therefore far less likely to cause a cough.

The cough incidence with ARBs is similar to placebo in clinical trials.

Switching from an ACE inhibitor:

If you develop a cough on an ACE inhibitor, your GP may switch you to losartan or another ARB.

NICE guidelines (NG136) recommend an ARB as an alternative first-line option for patients who cannot tolerate ACE inhibitors.

The cough usually resolves within 1 to 4 weeks of stopping the ACE inhibitor.

It is worth noting that a small proportion of patients (around 1 to 2%) may still develop a cough with losartan. If this occurs, discuss it with your doctor to rule out other causes.

Most side effects of losartan are mild and can be managed with simple measures.

For dizziness:

• Stand up slowly from sitting or lying positions
• Avoid driving or operating machinery until you know how losartan affects you
• Stay well hydrated, particularly in hot weather or during exercise
• Avoid alcohol in excess, which can worsen dizziness

For high potassium:

• Attend all blood test appointments scheduled by your GP
• Avoid potassium supplements, low-sodium salt substitutes (which contain potassium) and excessive consumption of potassium-rich foods such as bananas, oranges and dried fruit
• Inform your doctor if you are also taking spironolactone, amiloride or co-amilofruse

For fatigue:

• Allow 2 to 4 weeks for your body to adjust
• Maintain regular, moderate exercise
• Ensure you are eating and sleeping well

General advice:

• Take losartan at the same time each day for consistent blood pressure control
• Do not stop losartan without medical advice
• Keep follow-up appointments for blood pressure and blood test monitoring
• See our losartan NHS information page for further details on prescribing and availability

Contact your doctor or seek emergency help in the following situations.

Contact your GP or call 111 if you experience:

• Persistent dizziness, especially if it is getting worse
• Swelling of the ankles, feet or legs
• Unexplained nausea, loss of appetite or dark urine (possible liver problem)
• Muscle weakness, irregular heartbeat or tingling sensations (possible high potassium)
• A significant reduction in the amount of urine you pass

Call 999 or go to A&E if you experience:

• Swelling of the face, lips, tongue or throat (angioedema)
• Difficulty breathing or swallowing
• Fainting or collapse
• Chest pain or a very irregular heartbeat

The NHS recommends continuing losartan even if you feel well, as high blood pressure rarely causes symptoms but can lead to serious complications if left untreated.

If you think you are experiencing a side effect, your GP can adjust the dose or consider an alternative medicine.