Since depressed patients are liable to attempt suicide by self-poisoning, fatal poisoning from overdose is an important part of the overall assessment of any antidepressant drug. Because prescription figures for drugs are available in Britain, a Fatal Toxicity Index can be calculated in terms of deaths (from national mortality data) per million National Health Service prescriptions. This was carried out for the years 1975-1984 (Cassidy and Henry 1987), and has since been updated. The older tricyclic antidepressants as a group had a higher Fatal Toxicity Index than the mean for this drugs studied. Monoamine oxidase inhibitors had intermediate toxicity, and newer drugs introduced since 1973 had lower toxicity than the mean figure for all drugs, which was 34.9 deaths per million prescriptions. If drugs are equally effective clinically, serious consideration should be given to prescribing antidepressants with a lower fatal toxicity as a contribution to suicide prevention.