ADVERSE REACTIONS The incidence of adverse effects has been ascertained from controlled clinical studies conducted in the United States and is based on data from 859 patients who received guanabenz for up to 3 years. There is some evidence that the side effects are dose-related. The following table shows the incidence of adverse effects occurring in at least 5% of patients in a study comparing guanabenz acetate to placebo, at a starting dose of 8 mg b.i.d. Adverse Effect Placebo (%) n=102 guanabenz acetate (%) n=109 Dry mouth 7 28 Drowsiness or sedation 12 39 Dizziness 7 17 Weakness 7 10 Headache 6 5 In other controlled clinical trials at the starting dose of 16 mg/day in 476 patients, the incidence of dry mouth was slightly higher (38%) and that of dizziness was slightly lower (12%), but the incidence of the most frequent adverse effects was similar to the placebo-controlled trial. Although these side effects were not serious, they led to discontinuation of treatment about 15% of the time. In more recent studies using an initial dose of 8 mg/day in 274 patients, the incidence of drowsiness or sedation was lower, about 20%. Other adverse effects were reported during clinical trials with guanabenz but are not clearly distinguishable from placebo effects and occurred with a frequency of 3% or less: Cardiovascular-chest pain, edema, arrhythmias, palpitations. Gastrointestinal-nausea, epigastric pain, diarrhea, vomiting, constipation, abdominal discomfort. Central nervous system-anxiety, ataxia, depression, sleep disturbances. ENT disorders-nasal congestion. Eye disorders-blurring of vision. Musculoskeletal-aches in extremities, muscle aches. Respiratory-dyspnea. Dermatologic-rash, pruritus. Urogenital-urinary frequency, disturbances of sexual function (decreased libido, impotence). Other-gynecomastia, taste disorders. In very rare instances atrioventricular dysfunction, up to and including complete AV block, has been caused by guanabenz.