Methotrexate is injected 1 day a week, on the same day of the week. For instance, if you give yourself the first injection on a Saturday morning, you will repeat the injection the next Saturday morning, and every Saturday morning after that. Methotrexate injection is used in combination with other cancer treatments to treat gestational trophoblastic neoplasia and breast cancer in adults. Methotrexate injection is also used alone to treat squamous cell carcinoma of the head and neck in adults. Methotrexate is a medication commonly used in the treatment of Rheumatoid Arthritis, Psoriatic Arthritis, and other autoimmune conditions. It works by suppressing abnormal high activity of your immune system to reduce inflammation, swelling, and damage to your joints. Methotrexate Injection: learn about side effects, dosage, special precautions, and more on MedlinePlus Methotrexate Injection can cause embryo-fetal toxicity, including fetal death. For non-neoplastic diseases, Methotrexate Injection is contraindicated in pregnancy. Methotrexate injection is used alone or together with other medicines to treat several types of cancer, such as breast, head and neck, lung, blood, bone, lymph node, and uterus cancers. Methotrexate injection may be given by the intramuscular, intravenous or intra-arterial route. However, the preserved formulation contains Benzyl Alcohol and must not be used for intrathecal or.