Methotrexate levels, a guide to therapy?

Methotrexate (MTX) is being used with increasing frequency in the treatment of inflammatory arthritis. Seventy-one patients have been treated with MTX with only five ceasing treatment because of toxicity. From a study of the serum levels of this drug, we believe that 10 mg intramuscularly once per week should be an optimum method of treating arthritis. The measurement of a 24 hour serum level of MTX, which normally is less than 0.01 mumole/litre after 10 mg I.M., should aid in identifying those patients more at risk of developing toxicity.