Drug interactions
Losartan + Trimethoprim
Severity: Major
Losartan and Trimethoprim: major interaction.
Last updated Sources: UK SmPC
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Trimethoprim blocks epithelial sodium channels in the distal nephron, reducing potassium excretion, and losartan reduces aldosterone-mediated potassium excretion. Together they can cause clinically significant hyperkalemia, particularly in elderly patients, those with renal impairment or with high trimethoprim doses.
What this interaction means in plain words
Losartan and Trimethoprim can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Losartan belongs to Cardiovascular; Trimethoprim belongs to Anti Bacterial.
Why it happens
Additive reduction in renal potassium excretion via ENaC blockade (trimethoprim) and AT1 blockade (losartan).
What to do
Check serum potassium and creatinine within several days of starting trimethoprim or co-trimoxazole. Consider an alternative antibiotic in patients with renal impairment or baseline hyperkalemia.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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References
Other interactions of Losartan
- Aliskiren + Losartan Contraindicated
- Losartan + Sparsentan Contraindicated
- Amifostine + Losartan Major
- Amiloride + Losartan Major
- Amlodipine-benazepril + Losartan Major
- Amlodipine-hydrochlorothiazide-valsartan + Losartan Major
Other interactions of Trimethoprim
Frequently asked questions
Can I take Losartan with Trimethoprim?
MediHelp rates the interaction as major. Losartan and Trimethoprim can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Trimethoprim blocks epithelial sodium channels in the distal nephron, reducing potassium excretion, and losartan reduces aldosterone-mediated potassium excretion. Together they can cause clinically significant hyperkalemia, particularly in elderly patients, those with renal impairment or with high trimethoprim doses.
How serious is the interaction between Losartan and Trimethoprim?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Check serum potassium and creatinine within several days of starting trimethoprim or co-trimoxazole. Consider an alternative antibiotic in patients with renal impairment or baseline hyperkalemia.
What other medicines interact with Losartan?
MediHelp also lists interactions of Losartan with, for example, Aliskiren, Sparsentan, Amifostine. The full list is on the Losartan page.
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