Diarrhoea and constipation are common side effects of many drugs. In older persons, these iatrogenic consequences com-pound the increased prevalence of acute and chronic illness. An awareness of diarrhoea and constipation as complications of drug therapy in this age group and the use of preventative measures wherever practical may significantly improve the quality of life of the patient. It is important to understand the mechanisms whereby medications may cause diarrhoea and constipation and to periodically review drug use in the elderly. This second part of the review explains the mechanisms of drug-induced constipation and presents examples of drugs com-monly associated with this abnormality of bowel function. Aust J Hosp Pharm 2000; 30: 210-13. INTRODUCTION Constipation, perhaps more so than diarrhoea, is a sig-nificant problem in older persons. Both constipation and a troublesome consequence, spurious or overflow diarrhoea due to faecal impaction, can seriously affect the quality of life of patients and carers. Unrecognised spurious diarrhoea can result in unnecessary investiga-tions that are both expensive and uncomfortable to the patient. Most instances of constipation in older persons are secondary to drug side effects, rather than an illness altering gastrointestinal tract motility or age-related changes of the gastrointestinal tract. There is no strict definition of constipation. This reflects the considerable variation in normal bowel hab-it. Constipation may be described as a chronic condition of over six weeks duration with the passage of hard stools and/or a frequency of bowel action less than three times a week. 1 Some patients may use the term constipation to refer to straining during the passage of stools or a sense of incomplete evacuation (real or imagined). A US study of 209 elderly persons living at home showed that 30 per cent of men and 29 per cent of women reported that they