It is well known that methotrexate (MTX), used at high dosage in cancer patients, must not be combined with a non-steroidal anti-inflammatory drug (NSAID) because of high risk of side effects; prescribed at low dosage (< or = 15 mg per week) in rheumatoid arthritis patients, MTX is often combined with an NSAID. Some cases reported in the literature underline the potential toxicity of the association of low dose MTX with an NSAID, but most of the pharmacological studies do not confirm this hypothesis. Except for salicylates, NSAIDs do not affect the absorption, distribution, protein binding, area under the curve, half-life, or the elimination of MTX. Therefore, if necessary, MTX (< or = 15 mg per week) can be combined with an NSAID during the treatment of rheumatoid arthritis.