Reduced clearance and acute
methotrexate toxicity has been attributed to the
concurrent use of various
penicillins (amoxicillin, benzylpenicillin, carbenicillin, dicloxacillin, flucloxacillin, mezlocillin, oxacillin, phenoxymethylpenicillin, piperacillin, and ticarcillin) in a small number of case reports.
Serious interactions between
methotrexate and
penicillins are uncommon. Risk
factors are as yet unknown and even patients taking low doses of
methotrexate have been affected. Given the current evidence monitoring is advisable. One
recommendation is to carry out twice-weekly platelet and white cell counts for
2 weeks initially, with the measurement of
methotrexate levels if
toxicity is
suspected.