Both liquorice and thiazide diuretics have hypokalaemic effects. In spite of their prevalent use, there are no studies on hypokalaemia caused by the combination of liquorice and thiazides. We recruited 10 healthy volunteers in order to study the effects of 32g liquorice alone or in combination with 25mg hydrochlorothiazide a day for 2 weeks. The trial had a randomized, open and crossover design. During the liquorice phase, there were no changes in plasma potassium, sodium, creatinine, renin activity, serum aldosterone, blood pressure or heart rate. Weight tended to increase by 0.4 kg (70.2 to 70.6kg; p=0.056). During the liquorice-hydrochlorothiazide phase, the plasma potassium decreased by 0.32mmol/l (p=0.0015), plasma renin activity increased by 1.6 μg/l/h (p=0.0064) and the weight decreased by 0.9kg (70.5 to 69.6kg, p=0.0065). Twenty per cent of the subjects (2/10) became hypokalaemic during the combined liquorice-hydrochlorothiazide treatment. Furthermore, both subjects developed hypokalaemia within the first week of the combined treatment leading to premature discontinuation. The evaluation of liquorice consumption habits is warranted when initiating thiazide medications to avoid the excessive risk of hypokalaemia associated with the combined use of low-dose liquorice and thiazide diuretics.