A week of antibiotics — why do his muscles ache and his urine look dark?
Clarithromycin stops the liver clearing simvastatin. The statin builds up, and muscle starts to break down.
Select any card for the reasoning, what to watch for and an exam tip.
The puzzle
A man takes simvastatin after a heart attack. He is allergic to penicillin, so his chest infection is treated with clarithromycin. Four days later his thighs and shoulders ache, his legs feel weak, and his urine is dark.
The answer, step by step
Who is most at risk
- Older people.
- Kidney impairment, hypothyroidism or a high alcohol intake.
- A personal or family history of muscle disorders, or of muscle problems on a statin.
- Higher statin doses.
What to do
Key takeaway
Clarithromycin raises simvastatin exposure: the BNF says avoid the pair. Pause the statin for the course or choose another antibiotic, and tell anyone on a statin to report unexplained muscle pain.