Methotrexate is a medication that is commonly used to treat various autoimmune diseases in children, such as juvenile idiopathic arthritis (JIA) and psoriasis. However, like all medications, it can have side effects.

According to a study published in Pediatric Rheumatology, the most common side effects of methotrexate in children with JIA were gastrointestinal symptoms such as nausea, vomiting, and abdominal pain. Additionally, some children experienced mild liver function test abnormalities, which resolved with dose reduction or discontinuation of the medication. Rare side effects included cytopenias (low blood cell counts) and pneumonitis (inflammation of the lungs).

Another source, AboutKidsHealth, notes that methotrexate can also cause mouth sores, hair loss, and skin rash. Additionally, it can suppress the immune system, which may increase the risk of infections. Children taking methotrexate should avoid live vaccines and should be monitored closely for signs of infection.

It is important to note that the benefits of methotrexate often outweigh the risks of side effects. However, children taking this medication should be closely monitored by their healthcare provider and should report any side effects immediately.

Sources:
[1] https://ped-rheum.biomedcentral.com/articles/10.1186/1546-0096-11-23
[2] https://www.aboutkidshealth.ca/Article?contentid=185&language=English