Pemetrexed injection is used in combination with other chemotherapy medications as a first treatment for a certain type of non-small cell lung cancer (NSCLC) that has spread to nearby tissues or to other parts of the body. Pemetrexed injection is also used alone to treat NSCLC as ongoing treatment in people who have already received certain chemotherapy medications and whose cancer has not worsened and in people who could not be treated successfully with other chemotherapy medications. Pemetrexed injection is also in combination with another chemotherapy medication as a first treatment for malignant pleural mesothelioma (a type of cancer that affects the inside lining of the chest cavity) in people who cannot be treated with surgery. Pemetrexed is in a class of medications called antifolate antineoplastic agents.