Major
Clarithromycin × Sildenafil
Macrolide antibiotic×Phosphodiesterase type 5 inhibitors (PDE5)
Mechanism
Clarithromycin blocks CYP3A4 – the main sildenafil metabolic route. Sildenafil plasma levels rise 3- to 5-fold. The hypotensive effect intensifies; priapism and ischaemic optic neuropathy risks rise.
Symptoms
Sudden blood pressure drop: dizziness, syncope. Priapism – painful erection lasting more than 4 hours, requiring emergency care. Worsening of visual disturbances (cyanopsia) and tinnitus.
Management
During clarithromycin therapy, cap sildenafil at 25 mg (instead of the standard 50–100 mg) and limit to one dose every 24 hours. Alternative antibiotics: azithromycin or doxycycline.