During the open-label active run-in phase, a tolerated dose-escalation regimen (either escalating from 30 to 90 or 60 mg tid, or no escalation but fixed dose of 30 mg tid) will be established for each patient. Once the tolerated treatment regimen has been established, patients will then be randomised to the double-blind crossover treatment. Fipamezole and matched placebo tablets are compared in 3 crossover blocks, each block consisting of a total of 28 days: 12 days fipamezole and 12 days placebo in random order, separated by two days of washout. The patients will be randomly assigned to one of the two possible treatment sequences (fipamezole first followed by placebo or placebo first followed by fipamezole).