The evidence for an interaction between methotrexate and penicillins is limited to case reports and animal studies. So far, there is no conclusive evidence that any particular patient group is more at risk of an interaction occurring, and it does not appear to be dose related.
If concurrent use of methotrexate and a penicillin is required, patients should be carefully monitored during treatment. Monitoring may include measuring platelets and blood counts twice weekly for two weeks, with methotrexate levels being taken if the patient becomes symptomatic.
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