ACE inhibitors and trimethoprim: high potassium and kidney strain

The NHS describes ramipril as a medicine widely used to treat high blood pressure and heart failure that is also prescribed after a heart attack. It helps to prevent future strokes, heart attacks and kidney problems and works by widening blood vessels. Other ACE inhibitors, such as enalapril and lisinopril, work in a similar way.

Trimethoprim is an antibiotic. The NHS says it is used to treat and prevent urinary tract infections such as cystitis, and occasionally other infections such as chest infections and acne. For most infections you usually take it twice a day and feel better within a few days.

Because ACE inhibitors are usually taken long term, an antibiotic prescribed for a urine infection can easily land on top of one, often from a clinician who is not the person who prescribed the ACE inhibitor.

In the BNF interaction list for ramipril, the trimethoprim entry says that both ramipril and trimethoprim can increase the risk of nephrotoxicity, and that both can increase the risk of hyperkalaemia. It adds that hyperkalaemia is particularly notable when ACE inhibitors or angiotensin-II receptor antagonists are given with spironolactone or eplerenone.

Nephrotoxicity means harm to the kidneys, and hyperkalaemia means a higher than normal level of potassium in the blood. The BNF wording is that the risk can increase, not that either effect will occur, and the entry does not give an avoid instruction. The usual approach is awareness, a sensible duration of treatment and blood tests where needed.

The NHS trimethoprim page also lists a short set of medicines to check with a pharmacist or doctor about, including methotrexate, warfarin and digoxin. ACE inhibitors are not named there, which is a reminder that the NHS patient lists are short and are not complete. The BNF is the more specific reference for this pair.

The NHS trimethoprim page lists muscle weakness, an abnormal heartbeat or chest pains as signs of high potassium in the blood and says to call 999 or go to A&E now, and not to drive yourself. The NHS ramipril page lists kidney-related signs under serious side effects: swollen ankles, blood in your pee or not peeing at all, which should prompt a call to a doctor or 111.

The NHS ramipril page also says if you have severe watery diarrhoea or are being sick because of a stomach bug, tell your doctor, since you may need to stop ramipril for a while. A urinary infection with vomiting is a situation in which this advice is worth remembering.

• Muscle weakness or an abnormal heartbeat: emergency.

• Chest pain: emergency.

• Swollen ankles, blood in the urine or passing very little urine: call a doctor or 111.

• Being sick or having severe diarrhoea while on ramipril: tell your doctor.

Say you take an ACE inhibitor whenever an antibiotic is prescribed, and ask the pharmacist to check it. If you also take a potassium-sparing diuretic such as spironolactone, or a potassium supplement, say so because the BNF highlights hyperkalaemia in that setting. The NHS ramipril page lists spironolactone, triamterene, amiloride and potassium among medicines that can increase potassium.

Do not stop the ACE inhibitor or skip a course of antibiotic on your own. If a prescriber decides that bloods are needed after a few days of trimethoprim, attend promptly.

GP practices and pharmacists check interactions on prescriptions in the NHS, and out-of-hours services use the same references. Private doctors and online prescribers work from the same BNF, but the person prescribing needs to see your medicine list, so keep one to hand and show it each time.