Methotrexate is an oral or injectable prescription medication used to treat moderate-to-severe psoriasis, especially when psoriasis is widespread, difficult to control with topical treatments, or associated with psoriatic arthritis. It works by calming the immune system and slowing the overactive inflammation that drives psoriasis. Methotrexate is usually taken once weekly, often with folic acid to reduce side effects. It may be a useful option when creams, ointments, or phototherapy are not enough, but it requires regular blood tests to monitor the liver, blood counts, and kidney function. Possible side effects include nausea, mouth sores, fatigue, low blood counts, liver irritation, and increased infection risk. Methotrexate should not be used during pregnancy or when trying to conceive unless specifically directed by a clinician.