A patient on methotrexate for psoriasis develops mouth ulcers; which adverse effect is most likely?

• A. Mouth ulcers
• B. Nausea
• C. Hair loss
• D. Skin rash

Answer: (A)

Metho trexate inhibits dihydrofolate reductase, reducing folate availability for DNA synthesis. This largely affects rapidly dividing cells, including the mucosal lining of the mouth, so mucositis with mouth ulcers is a common adverse effect even at the low, weekly doses used for psoriasis. While other adverse effects like nausea, hair loss, or rash can occur, the mouth ulcers directly reflect the mucosal toxicity caused by methotrexate. To help prevent this, folic acid supplementation is often used alongside therapy, and dose adjustments or monitoring for broader toxicities may be needed if mucositis or other adverse effects worsen.