Certainly there can be additive toxicities of many medications. In the case of methotrexate and Arava (leflunomide) these can be effects on the liver and on blood counts. However, there are also well documented reports of the efficacy of this combination. In general, when used in combination the blood is monitored very carefully and the doses of the agents are sometimes decreased to allow the combination to be given.
About Clifton Bingham, III, MD
Professor of Medicine
Director - Johns Hopkins Arthritis Center