ObjectivesHigh potassium intake could prevent stroke, but supplementation is considered hazardous. We assessed the effect of oral potassium supplementation on serum or plasma potassium levels and renal function.SettingWe updated a systematic review of the effects of potassium supplementation in randomised clinical trials carried out worldwide, published in 2013, extending it to July 2015. We followed the PRISMA guidelines.ParticipantsAny individual taking part in a potassium supplementation randomised clinical trial. Studies included met the following criteria: randomised clinical trials, potassium supplement given and circulating potassium levels reported.InterventionOral potassium supplementation.Primary outcome measuresSerum or plasma potassium and serum or plasma creatinine.ResultsA total of 20 trials (21 independent groups) were included (1216 participants from 12 different countries). All but 2 were controlled (placebo n=16, control n=2). Of these trials, 15 were crossover, 4 had a parallel group and 1 was sequential. The duration of supplementation varied from 2 to 24 weeks and the amount of potassium given from 22 to 140 mmol/day. In the pooled analysis, potassium supplementation caused a small but significant increase in circulating potassium levels (weighted mean difference (WMD) 0.14 mmol/L, 95% CI 0.09 to 0.19, p<1×10−5), not associated with dose or duration of treatment. The average increase in urinary potassium excretion was 45.75 mmol/24 hours, 95% CI 38.81 to 53.69, p<1×10−5. Potassium supplementation did not cause any change in circulating creatinine levels (WMD 0.30 µmol/L, 95% CI −1.19 to 1.78, p=0.70).ConclusionsIn short-term studies of relatively healthy persons, a moderate oral potassium supplement resulted in a small increase in circulating potassium levels and no change in renal function.