1. Boston Collaborative Drug Surveillance Program, Boston University School of Medicine, 11 Muzzey Street, Lexington, MA 02421, USA
2. Basel Pharmacoepidemiology Unit, Division of Clinical Pharmacology and Toxicology, University Hospital, Spitalsstrasse 26, 4031 Basel, Switzerland
3. Castlemilk Group Practice, Dougrie Drive, Glasgow G45 9AW, UK