The fluoropyrimidine 5-fluorouracil (5-FU), and its prodrugs capecitabine and tegafur, can be used for treating cancers, including gastrointestinal, breast, and head and neck cancer. Adverse reactions to fluoropyrimidines ranging from diarrhoea to bone marrow suppression (myelosuppression) occur in 10-40 per cent of patients but can be fatal in 1 per cent of cases.¹

The enzyme dihydropyrimidine dehydrogenase (DPD), which inactivates most of a dose of 5-FU, is encoded for by the DPYD gene. At least four DPYD gene variants have reduced activity, meaning less DPD is present thereby increasing the risk of 5-FU toxicity. Testing for DPYD activity was introduced into the NHS two years ago, with a subsequent reduction in serious ADRs.