For a patient taking methotrexate, it is best to discontinue the methotrexate before starting antibiotics as these two drugs work at cross purposes.
Tetracyclines destroy bowel flora which are needed to break down the methotrexate.
This may result in increased free methotrexate concentrations. The toxic side effects of methotrexate may be increased if this drug is combined with certain drugs such as tetracyclines. These drugs may inhibit the elimination of methotrexate and may allow a build up in the bladder and kidneys which increases the chance of toxic side effects. Because the methotrexate is eliminated through the kidneys and liver, a drug build up may cause a problem, especially in patients whose kidney and or liver function is already impaired. These problems may not show up right away.