Two multicenter randomised, double-blind, placebo-controlled trials were carried out to determine the short-term and long-term effects of dichlorphenamide (DCP) on attack frequency and quality of life in hyperkalemic (HYP) and hypokalemic (HOP) periodic paralysis. The trials, which included 44 HOP and 21 HYP participants, lasted 9 weeks and were followed by a 1-year extension phase in which all participants received DCP. The primary outcome variable was the average number of attacks per week over the final 8 weeks of the double-blind phase. The median attack rate was lower in HOP participants on DCP than in participants on placebo (0.3 vs 2.4, p = 0.02). The 9-week mean change in the Physical Component Summary score of the Short Form-36 was also better in HOP participants receiving DCP (treatment effect = 7.29 points, 95% confidence interval 2.26 to 12.32, p = 0.006). The median attack rate was also lower in HYP participants on DCP (0.9 vs 4.8) than in participants on placebo, but the difference in median attack rate was not significant (p = 0.10). There were no significant effects of DCP on muscle strength or muscle mass in either trial. The most common adverse events in both trials were paresthesia (47% DCP vs 14% placebo, both trials combined) and confusion (19% DCP vs 7% placebo, both trials combined). DCP is effective in reducing the attack frequency, is safe, and improves quality of life in HOP periodic paralysis.