methotrexate and naproxen

Naproxen, a nonsteroidal anti-inflammatory drug (NSAID), inhibits renal prostaglandin synthesis, leading to reduced renal blood flow and decreased renal clearance of methotrexate. Additionally, naproxen may displace methotrexate from plasma protein binding, increasing the concentration of free (unbound) methotrexate. These effects elevate methotrexate plasma levels, increasing the risk of severe toxicity.

Co-administration can result in methotrexate toxicity, manifested as myelosuppression (pancytopenia), mucositis, hepatotoxicity, and acute kidney injury. Patients may present with fever, sore throat, oral ulcers, easy bruising, or bleeding. Fatalities have been reported, particularly in patients with impaired renal function or high-dose methotrexate therapy.

Avoid concurrent use of methotrexate and naproxen, particularly in patients with renal impairment or those receiving high-dose methotrexate. If co-prescription is unavoidable, use the lowest effective doses and monitor closely. Monitor full blood count, renal function, and liver function at baseline and at regular intervals during therapy. Consider measuring serum methotrexate levels where available. Administer methotrexate with folic acid supplementation to reduce some toxicities. Advise patients to report signs of toxicity such as oral ulcers, fever, or easy bruising.

This interaction is well established through pharmacokinetic studies and numerous case reports. The risk is higher with high-dose methotrexate and in patients with reduced renal function. Guidelines recommend avoiding NSAIDs, including naproxen, in patients on methotrexate, unless careful monitoring is possible.