Adverse Drug Reaction Reporting and Causality Assessment for a Suspected Statin Myopathy

Likely diagnosis: Statin-induced rhabdomyolysis - proximal myopathy, markedly elevated CK (~24× upper limit), dark urine (myoglobinuria), and acute kidney injury.

Mechanism: Simvastatin is metabolised primarily by CYP3A4. Clarithromycin is a potent CYP3A4 inhibitor (and also inhibits OATP1B1/transporters). Co-administration markedly raises simvastatin plasma concentrations, increasing the risk of myotoxicity. This is a well-recognised, potentially fatal interaction. The temporal relationship (clarithromycin started 4 weeks ago, symptoms over 3 weeks) fits.

Contributing factors: age, diabetes, possible renal impairment, and the high simvastatin dose (40 mg is the maximum recommended with interacting drugs - but with a strong CYP3A4 inhibitor, simvastatin should be avoided entirely).

Immediate action: Stop simvastatin now; this is a medical emergency requiring urgent assessment (CK, renal function, urine output, electrolytes for hyperkalaemia). The patient should be referred to the emergency department - dark urine and rising creatinine indicate rhabdomyolysis with AKI.

Causality assessment (e.g., using WHO-UMC or Naranjo criteria):

• Temporal relationship: plausible - clarithromycin started 4 weeks ago, symptoms 3 weeks ago.
• Known reaction: yes - statin myopathy/rhabdomyolysis with CYP3A4 inhibitors is well documented.
• Dechallenge: positive once statin stopped.
• Alternative causes: hypothyroidism, strenuous exercise, other myotoxic drugs, renal impairment - should be excluded.
• Rechallenge: not appropriate.

Likely 'probable' or 'highly probable' on WHO-UMC criteria.

Reporting:

• Report to the national pharmacovigilance authority (in the UAE, the MOHAP pharmacovigilance system / ADR reporting form) as a suspected serious adverse drug reaction - seriousness criteria: life-threatening/hospitalisation, significant disability, or renal injury.
• The prescriber should also report; pharmacists can and should report directly.
• Include: patient demographics (anonymised), suspect drug(s) and dose, concomitant drugs (clarithromycin), dates, reaction description, CK/creatinine values, outcome, and reporter details.

Follow-up: report outcome after recovery; flag the interaction in the patient's record and counsel him to avoid simvastatin with strong CYP3A4 inhibitors in future (e.g., use pravastatin or rosuvastatin, which are less CYP3A4-dependent, if a statin is needed with such agents).

Patient counselling:

• Stop simvastatin immediately and go to the emergency department now - dark urine and rising creatinine mean this is urgent, not a wait-and-see problem.
• Explain the interaction in plain language: 'the antibiotic you were given raised the level of your cholesterol medicine to a dangerous level, which damaged your muscles.'
• Do not restart any statin until reviewed; future statin choice must avoid strong CYP3A4 inhibitors (or use a statin less dependent on CYP3A4, e.g., pravastatin or rosuvastatin, with appropriate monitoring).
• Report any recurrence of muscle pain, weakness, or dark urine promptly.

System-level prevention:

• Pharmacy alert / interaction checking at the point of dispensing - clarithromycin + simvastatin should trigger a hard stop or pharmacist review.
• Prescriber education and electronic prescribing alerts.
• Medication reconciliation at every transition, including walk-in clinics.
• Patient-held medication record so any prescriber sees the full list.
• Pharmacovigilance feedback loop: report, and share learning locally so the same interaction is caught next time.