( 7.1 , 7.2 ) Trimethoprim-Sulfamethoxazole : Avoid concomitant use of leucovorin calcium tablets with trimethoprim-sulfamethoxazole. Trimethoprim-Sulfamethoxazole Avoid concomitant use of leucovorin calcium tablets with trimethoprim-sulfamethoxazole. The effectiveness of trimethoprim-sulfamethoxazole can be decreased if used concomitantly with leucovorin calcium tablets, which was associated with increased rates of treatment failure and mortality in patients with HIV infection who receive trimethoprim-sulfamethoxazole for the acute treatment of Pneumocystis jirovecii pneumonia.
Source: FDA drug label - leucovorin calcium