Trimetrexate is primarily used as an antiparasitic and anticancer agent. It works by inhibiting dihydrofolate reductase, an enzyme crucial for DNA synthesis, making it effective in targeting rapidly dividing cells. In clinical settings, it is employed to treat Pneumocystis pneumonia (PCP), particularly in patients with compromised immune systems, such as those with HIV/AIDS. Additionally, it has been investigated for its potential in treating certain cancers, including leukemia and solid tumors, due to its ability to disrupt cell proliferation. Its use often requires concurrent administration of leucovorin to protect normal cells from its cytotoxic effects.