The authors report adverse events (AEs) related to the use of chlorhexidine gluconate mouthrinse in a clinical trial of the efficacy of periodontal treatment in older adults with diabetes. Participants were U.S. veterans with uncontrolled diabetes (hemoglobin A(1c) value > or =8.5 percent) and periodontal disease. Treatment included periodontal scaling, 0.12 percent chlorhexidine lavage during ultrasonic scaling and use of chlorhexidine mouthrinse at home. Forty-four (31 percent) of 140 subjects reported having AEs. Most common were taste changes and tooth staining, sore mouth and/or throat, tongue irritation and wheezing/shortness of breath; the latter was reported more commonly before chlorhexidine use than after. Only body mass index greater than 30 was significantly related to AEs. AEs are common among subjects using chlorhexidine mouthrinse. Most AEs (taste change and staining) were resolved easily by subjects' discontinuing mouthrinse use and receiving dental prophylaxis. No serious AEs were reported. Clinicians should advise patients using chlorhexidine mouthrinse of possible side effects. If necessary, patients should discontinue mouthrinse use and obtain medical care. Careful monitoring of AEs in patients using chlorhexidine is warranted.