Pemetrexed disodium salt 2.5 hydrate is primarily used in the treatment of certain types of cancer, particularly non-small cell lung cancer and mesothelioma. It functions as an antimetabolite, inhibiting the enzymes involved in the synthesis of nucleotides, which are essential for DNA replication and cell division. This action helps to slow or stop the growth of cancer cells. It is often administered in combination with other chemotherapy agents, such as cisplatin, to enhance its efficacy. The drug is typically given intravenously under medical supervision, and patients may require pre-treatment with vitamins like folic acid and vitamin B12 to reduce potential side effects. Regular monitoring of blood counts and kidney function is necessary during treatment to manage any adverse effects effectively.