Alimta is a medication used to treat cancer, which may be referred to as a chemotherapy medication, a cytotoxic agent or an antineoplastic agent. It is used to treat two types of lung cancer, mesothelioma and non-small cell lung cancer. It contains an active ingredient called pemetrexed. This ingredient affects the action of enzymes found in cells, which stop the multiplication of, or slow the growth of cancer cells.