I realize this answer is very, very late ... but may help someone in future.
There is potential for a serious interaction between methotrexate and amoxicillin. The interaction would mainly be of concern if the patient was taking a large dose of methotrexate and/or was on long term amoxicillin.
MONITOR CLOSELY: Concomitant use of large doses of penicillins may elevate serum methotrexate concentrations. The mechanism may involve competitive inhibition of renal tubular secretion of methotrexate. In one study, methotrexate clearance reductions were 35%, 40%, and 66% with penicillin, ticarcillin, and piperacillin, respectively. Serious adverse effects, primarily hematologic, have been described with concurrent penicillins and high or low-dose methotrexate regimens. Fatalities have occurred with both types of methotrexate regimens.
MANAGEMENT: If a penicillin must be used concurrently, close monitoring of methotrexate serum concentrations and of the patient for evidence of serious methotrexate toxicity are recommended. Leucovorin rescue should be available. Methotrexate dose reductions may be necessary. If broad-spectrum antibiotic coverage is needed during high-dose methotrexate therapy, use of other antimicrobials may be preferable. Patients should be advised to promptly report symptoms including fever, chills, sore throat, bruising, bleeding, stomatitis, or malaise to their physician.
- Methotrexate Information for Consumers
- Methotrexate Information for Healthcare Professionals (includes dosage details)
- Side Effects of Methotrexate (detailed)
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