Penicillins

This information on drug interactions with antimicrobials is intended for use as a guide and not as a complete reference source

Antibiotic Class

Interacting Drug

Comment

Penicillins e.g.

Amoxicillin
Co-amoxiclav
Flucloxacillin
Phenoxymethylpenicillin

Methotrexate

Increased risk of toxicity with methotrexate: careful monitoring for signs and symptoms of methotrexate toxicity (i.e. haematological and gastrointestinal toxicity) particularly in patients with renal impairment and the elderly.

Warfarin

Penicillins and cephalosporins are preferred alternatives when patients are on warfarin. Documented reports of bleeding incidents with these antibiotics are rare even though a theoretical risk exists. Monitor INR during and after antimicrobial treatment and adjust warfarin dose as necessary.