Spironolactone and Co-trimoxazole: Check the New Prescription

If you take spironolactone and receive co-trimoxazole, ask the prescriber or pharmacist to check the combination before taking them together. The current UK spironolactone product information warns that co-trimoxazole, also called trimethoprim-sulfamethoxazole, can increase blood potassium when combined with it. The medical term is hyperkalaemia.¹

Why does a short antibiotic course matter?

The interaction warning concerns the medicines taken together, even when spironolactone is an established prescription. Other potassium-raising products also matter, including potassium supplements, potassium-containing salt substitutes, some blood-pressure medicines and anti-inflammatory painkillers. Give the pharmacist the whole list.¹

If spironolactone was prescribed for a hair or skin concern, include it when discussing a UTI just as you would a heart or blood-pressure medicine. The reason you take it does not remove it from the interaction check.

What did the study actually measure?

A 2011 Ontario study examined people aged 66 or older taking spironolactone. Researchers compared antibiotic prescriptions in people hospitalized with hyperkalaemia within 14 days of an antibiotic prescription and matched controls from the same spironolactone cohort. They analyzed 248 cases and 783 controls.²

Compared with amoxicillin, trimethoprim-sulfamethoxazole was associated with 12.4 times the adjusted odds of hospitalization for hyperkalaemia, with a 95% confidence interval of 7.1 to 21.6. That is an odds comparison, not a claim that 12.4% of people will be hospitalized.²

The study was observational. It used administrative records and lacked blood potassium results, detailed kidney-function measurements and information about whether patients took the dispensed medicines. Its authors cautioned that findings might not apply to younger people with fewer risk factors. It does not supply a personal risk percentage for a healthy younger woman.²

Amoxicillin was the comparison drug, not a recommendation that every UTI should be treated with it. Antibiotic selection belongs with the clinician treating the infection.

What should I say when I call?

Have both packets available and say:

“I take spironolactone, at this dose, for this reason. I have now been prescribed co-trimoxazole. Have you checked the combination? Do I need a different antibiotic, or potassium and kidney-function blood tests? Please tell me what to take and when.”

If you have already taken both, give the doses and times and request a prompt review. Ask for instructions about the next doses rather than inventing a schedule yourself. Mention known kidney problems and every supplement, including products bought for joint health or general wellness. Medsafe’s³ 2024 safety update describes potassium-containing supplements as possible contributors to hyperkalaemia.³

Can I rely on feeling well?

High potassium often has no symptoms. Medsafe³ lists possible symptoms including muscle weakness, nausea, vomiting and palpitations, but their absence does not check your potassium level. Discuss the blood-test plan with the clinician rather than waiting for symptoms to decide whether the combination needs attention.³

The medication review should also address the infection. In the UK, NHS⁴ guidance advises an urgent GP appointment or NHS 111 for suspected UTI at age 65 or older, or with fever, shivering, blood in the urine or pain under the ribs in the back. Confusion, drowsiness or difficulty speaking require 999 or A&E. Elsewhere, use the corresponding urgent or emergency service.⁴

Hair, Everywhere discusses bringing medicines and supplements into the appointment in Chapter 3. Keep that list available when a different clinician adds a prescription. Our spironolactone prescribing checklist covers the broader review.