nitrofurantoin + methotrexate

if oral methotrexate use, consider alternative or monitor CBC, renal fxn: combo may incr. methotrexate levels, risk of serious infection, myelosuppression, nephrotoxicity, other adverse effects; may decr. methotrexate active metabolite levels, efficacy (decreased absorption; intestinal metabolism inhibited, decr. conversion to active metabolite)

sulfasalazine + methotrexate

monitor CBC: combo may incr. risk of serious infection, myelosuppression (additive effects)

sulfasalazine + nitrofurantoin

caution advised: combo may incr. risk of methemoglobinemia (additive effects)