Leucovorin is used as an antidote for toxicity of which agent?

• A. Methotrexate
• B. Warfarin
• C. Benzodiazepines
• D. Thallium

Answer: (A)

Leucovorin acts as a rescue after methotrexate exposure by supplying reduced folate (folinic acid) that bypasses the block in dihydrofolate reductase. This lets normal cells continue thymidylate and purine synthesis, reducing methotrexate toxicity while allowing the drug to exert its effect on cancer cells when timing is carefully managed.

It's not used for reversing warfarin toxicity—that role is fulfilled by vitamin K. It isn’t a general antidote for benzodiazepines or thallium poisoning, which are managed with different treatments or chelation strategies.