Folinic acid is an approved drug used to protect against the side-effects of folate antagonist drugs such as trimetrexate. Cells that absorb folinic acid are protected against trimetrexate, while those that do not absorb folinic acid can be seriously damaged by trimetrexate. Since folinic acid is taken up by human cells but not by, for example, Pneumocystis jiroveci organisms, the combination of folinic acid with trimetrexate allows the effects of the trimetrexate to be targeted against Pneumocystis.

Folinic acid needs to be continued for 72 hours after the last dose of trimetrexate to ensure adequate protection. Without folinic acid, trimetrexate can cause serious bone marrow suppression and kidney or liver damage.

Folinic acid may also be prescribed for people who are taking other drugs that can interfere with folate metabolism, such as intravenous co-trimoxazole.

The drug comes in the form of tablets or a liquid for intravenous injection.

Folinic acid is also known as leucovorin, calcium leucovorin or calcium folinate. It is available as a generic (non-brandname) product and is also manufactured by Pharmacia & Upjohn under the tradename Refolinon.