Drug interactions
Fosinopril + Trimethoprim
Severity: Major
Fosinopril 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 ACE inhibitors also raise potassium. Together they can cause clinically important hyperkalemia, especially in older patients and those with renal impairment.
What this interaction means in plain words
Fosinopril 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.
Fosinopril belongs to Cardiovascular; Trimethoprim belongs to Anti Bacterial.
Why it happens
Additive reduction in renal potassium excretion through ENaC blockade by trimethoprim and aldosterone suppression by ACE inhibition.
What to do
Avoid the combination in high-risk patients where an alternative antibiotic exists. If used, check serum potassium and creatinine within several days and again as needed.
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 Fosinopril
- Aliskiren + Fosinopril Contraindicated
- Fosinopril + Protein a column Contraindicated
- Fosinopril + Sacubitril / valsartan Contraindicated
- Fosinopril + Sacubitril-valsartan Contraindicated
- Amifostine + Fosinopril Major
- Amiloride + Fosinopril Major
Other interactions of Trimethoprim
Frequently asked questions
Can I take Fosinopril with Trimethoprim?
MediHelp rates the interaction as major. Fosinopril 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 ACE inhibitors also raise potassium. Together they can cause clinically important hyperkalemia, especially in older patients and those with renal impairment.
How serious is the interaction between Fosinopril 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: Avoid the combination in high-risk patients where an alternative antibiotic exists. If used, check serum potassium and creatinine within several days and again as needed.
What other medicines interact with Fosinopril?
MediHelp also lists interactions of Fosinopril with, for example, Aliskiren, Protein a column, Sacubitril / valsartan. The full list is on the Fosinopril page.
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