Myotonia is an abnormal delay in the relaxation of muscles after contraction. It is a key symptom in a number of muscle diseases called myotonic disorders. It can be mild or severe, interfering with daily activities such as walking, climbing stairs or opening and closing the eyelids. It can be worse after periods of rest or triggered by cold or fatigue. People with mild myotonia can manage their disease without medication but in severe cases treatment is usually necessary. Drugs that have been used to treat myotonia include sodium channel blockers such as procainamide, phenytoin and mexiletine, tricyclic antidepressant drugs such as clomipramine or imipramine, benzodiazepines, calcium antagonists, taurine and prednisone. This review describes ten randomised controlled trials which tested the effectiveness of twelve different drug treatments. The review was updated in July 2009 and no new trials were found. The ten trials included a total of 143 participants of which 113 had myotonic dystrophy and 30 had myotonia congenita. The trials were generally small and of poor quality. Meta-analysis was not possible due to a lack of appropriate trials and data. Two small studies suggested that clomipramine and imipramine might have a short-term beneficial effect on the myotonia in myotonic dystrophy and one small study suggested that taurine might have a long-term beneficial effect in myotonic dystrophy. Minor side effects such as dry mouth and dizziness were reported with clomipramine and imipramine, but not with taurine. It was not possible to determine whether drug treatment is safe and effective for myotonia in people with a myotonic disorder based on the evidence from the ten trials included in this review. Larger, well-designed randomised controlled trials are needed.