Women taking cyclosporin are not advised to breast feed because cyclosporin is excreted in milk at concentrations similar to those in blood.1 Every case report2 and the manufacturer3 advises women not to breast feed. We report the case of a woman taking cyclosporin who successfully breast fed her baby.

A woman who had received a renal transplant was keen to breast feed her baby but was advised not to because she was taking cyclosporin. None the less, she breast fed initially. At 5 weeks her daughter was well, with normal renal function and a blood cyclosporin concentration of no more than 3 μg/l; the mother's simultaneous trough concentration was 260 μg/l while receiving a dose of 3 mg/kg twice daily. Milk cyclosporin concentrations varied with time after the dose, averaging 596 μg/l. A baby taking 150 ml of milk per kilogram weight every day would receive <0.1 mg/kg of cyclosporin—under a 60th of his or her mother's dose, weight for weight. This woman breast fed her daughter fully until weaning and then partially until she was 14 months old. Her kidney transplant function remained stable, and her daughter was healthy at 2 years old. The patient then breast fed her son.

A mother receiving drug treatment who has been counselled against breast feeding will reasonably assume that the hazards of a drug have been balanced against the disadvantages of formula feeding. This is not so for cyclosporin.

Babies are thought to receive a large dose of cyclosporin because blood and milk concentrations are similar,2 3 but this is not so and is just one of many pharmacological factors. Although fetuses may be exposed to blood cyclosporin concentrations that are about one third of maternal amounts,2 no adverse effects have been described; risks from the much lower quantities during breast feeding are likely to be minimal.

Breast feeding appreciably reduces the risk of infant infections and admission to hospital,4 and being breast fed is associated with significantly better intellectual development,5 but published work does not mention the losses to the baby of being fed on formula milk.2 3 Human milk confers major benefits; advice on breast feeding should balance the measured risk from maternal drugs with the undoubted disadvantages of formula feeding.

We thank Dr David Holt, Cardiological Sciences Analytical Unit, St George's Hospital Medical School, London, for measuring the baby's blood cyclosporin concentration.