Based on NIH | Can losartan cause fatigue, and when should I contact my doctor if I feel unusually tired while taking it?

Losartan can cause fatigue in some people, though it is generally well‑tolerated and fatigue tends to be mild and uncommon. In clinical trials, “asthenia/fatigue” was reported in a small percentage of users and also occurred at similar rates with placebo, which suggests it is not unique to losartan but can happen. ^{[1] [2]} If your tiredness is severe, new, or persistent, or if it comes with dizziness, fainting, chest pain, swelling, or signs of infection, you should contact your doctor. ^{[3] [4]}

What the evidence shows

• Fatigue has been observed among common adverse experiences in trials of losartan, with rates roughly comparable to placebo. ^{[1] [2]}
• Dizziness is one of the more frequent side effects considered related to the drug, and low blood pressure can make you feel faint or weak. ^{[1] [3]}
• Consumer and professional medication guides list fatigue/asthenia among reported adverse reactions, especially when losartan is used alongside other blood pressure therapies. ^{[3] [5]}

Why losartan might make you feel tired

• Blood pressure lowering: If your blood pressure drops too much, you may feel weak, lightheaded, or fatigued, particularly after starting or changing dose. ^[3]
• Electrolytes and kidneys: In some cases, losartan can affect potassium levels (hyperkalemia) or kidney function, which can contribute to fatigue. ^[3]
• Illness overlap: Colds and upper respiratory symptoms are common while on losartan and can themselves cause tiredness. ^[4]

When to seek medical advice

You might consider watchful waiting if your fatigue is mild and improves as your body adjusts, but there are clear times to call your doctor:

• You feel faint or very dizzy, or you actually faint. ^{[3] [4]}
• Fatigue is persistent, worsening, or is limiting daily activities. ^[3]
• You notice swelling in feet, ankles, or hands, or unexplained weight gain, which can signal fluid retention or kidney issues. ^[4]
• You develop chest pain, shortness of breath, or new muscle weakness. ^[3]
• You have nausea/vomiting with reduced fluid intake, diarrhea, or any illness that could lead to dehydration, which may intensify low blood pressure. ^[6]

Red‑flag symptoms that need urgent care

• Signs of an allergic reaction (swelling of face, lips, throat, or tongue). ^[7]
• Severe lightheadedness with near‑syncope or syncope. ^{[3] [4]}

Practical steps you can try

• Check your blood pressure at home: If readings are very low for you (for example, systolic below 90 mmHg or symptomatic), discuss dose adjustment. ^[3]
• Rise slowly: Sit or stand up gradually to reduce orthostatic dizziness that can feel like fatigue. ^[3]
• Hydrate and avoid excess alcohol: Dehydration can worsen low blood pressure and tiredness. ^[6]
• Review other medicines: Combining losartan with diuretics or other antihypertensives can heighten low blood pressure and fatigue ask your clinician if a timing or dose change might help. ^[3]
• Report new swelling or weight changes promptly to check kidney function and electrolytes. ^[4]

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Quick reference: common and notable effects

Item	What to know	Action
Fatigue/asthenia	Reported in trials; often mild and similar to placebo rates. [1] [2]	Monitor; call if persistent or worsening. [3]
Dizziness/faintness	More often considered drug‑related; can be due to low blood pressure. [1] [3]	Lie down; rise slowly; call doctor if fainting or severe. [3] [4]
Swelling/weight gain	May indicate kidney issues; needs evaluation. [4]	Call your doctor. [4]
Chest pain or muscle weakness	Reported post‑marketing; requires assessment. [3]	Call your doctor. [3]
Infection‑like symptoms	“Colds” are common and can increase tiredness. [4]	Supportive care; call if severe or persistent. [4]

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Bottom line

Fatigue can occur with losartan, but it is usually mild and not unique to this medication. ^{[1] [2]} If you feel unusually tired especially with dizziness, fainting, swelling, or other concerning symptoms reach out to your doctor to review your dose, check blood pressure, and consider labs for kidney function and potassium. ^{[3] [4]}