Leucovorin is a derivative of folic acid. It comes in either an IV (intravenous) or oral form.

Leucovorin is used either with methotrexate for the treatment of ectopic pregnancy or with fluorouracil (5-FU) for advanced colon cancer. Leucovorin works differently, depending on which medicine it is used with.

• Methotrexate: Leucovorin is given to protect the gastrointestinal lining and bone marrow cells from methotrexate toxicity. Most of the complications and side effects of methotrexate can be either prevented or treated by using leucovorin, which is usually given 24 hours after methotrexate.
• Fluorouracil: Leucovorin increases the effectiveness of 5-FU. This results in a greater anticancer effect.

Leucovorin is very well tolerated and has almost no side effects of its own. It generally decreases the severity of methotrexate side effects. When used with 5-FU, however, leucovorin can increase the severity of its side effects.