How do folic acid and methotrexate interact?

Methotrexate inhibits the enzyme (called dihydrofolate reductase) that folic acid requires to become active in the body. However, folic acid may decrease side effects such as mouth sores in patients who take methotrexate for arthritis and other non-malignant diseases. 

Methotrexate treats many diseases. These include:

• Rheumatoid arthritis
• Lupus
• Psoriasis (red areas of skin covered by silvery, scaly patches)
• Asthma
• Sarcoidosis  (an autoimmune inflammatory disease)
• Inflammatory bowel disease
• Cirrhosis (liver scarring)