In 1995 a report in the New England Journal of Medicine on medication use and risk of Stevens-Johnson syndrome/ toxic epidermal necrolysis (SJS/TEN) identified carbamazepine as a drug with a higher excess risk than most other drugs known to be associated with this severe muco-cutaneous drug reaction. Since then, pharmacogenomics (the study of the role of inherited and acquired genetic variation in drug response) has emerged as a new discipline, which is increasingly impacting on routine clinical practice. Perhaps the best known example of pharmacogenomics is variable thiopurine methyl transferase (TPMT) activity and the effect it has on azathioprine metabolism. Indeed, it is now the norm in the UK to measure TPMT activity before prescribing azathioprine in order to detect patients at risk of toxicity from conventional doses of this widely used drug. More recently, clinically relevant pharmocogenomic features have been identified for warfarin and clopidogrel.