doxycycline + methotrexate

This is a combination of two pharmaceutical drugs, doxycycline and methotrexate. Doxycycline is a tetracycline-class antibiotic that inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit, used for various bacterial infections. Methotrexate is an antimetabolite and antifolate agent that inhibits dihydrofolate reductase, interfering with DNA synthesis, repair, and cellular replication; it is used in cancer chemotherapy as well as autoimmune diseases such as rheumatoid arthritis. When administered together, there is evidence of a clinically significant drug-drug interaction—doxycycline can increase plasma concentrations of methotrexate by displacing it from protein-binding sites or affecting its renal clearance. This may lead to increased risk of methotrexate toxicity (hematologic and gastrointestinal), especially at high doses[1][2][3]. The combination should be used with caution due to these pharmacokinetic interactions.