Adverse treatment effects in the trial of the European Working Party on High Blood Pressure in the Elderly. Adverse treatment effects were assessed in 840 elderly hypertensive patients randomly assigned to active treatment (a combination of triamterene and hydrochlorothiazide) or placebo; methyldopa was added to the regimen in one third of the treated patients. Symptoms of dry mouth, nasal stuffiness, and diarrhea were reported by significantly more treated patients than placebo control subjects. More patients receiving diuretics plus methyldopa than diuretics alone reported dry mouth and diarrhea. Significantly more treated patients than control subjects showed evidence of a high serum creatinine level, mild hypokalemia, and gout. More treated patients tended to have diabetes. The benefits of treatment outweighed these adverse treatment effects. 