According to the UK's National Health Service, weight gain is not a typical side effect of the rheumatoid arthritis medication methotrexate. Corticosteroids, such as prednisolone, are also prescribed for RA, and these sometimes cause patients to put on weight.
Although doctors often start RA patients off on methotrexate, the drug has many possible side effects, NHS reports. MedlinePlus indicates that joint or muscle pain, red eyes, swollen gums or hair loss should be reported to a doctor if it is severe or persistent. Some side effects require immediate medical attention. These include vomiting, vision problems, fever, headache, seizures, confusion, body weakness, difficulty walking, hives and swelling of the face or extremities.