OBJECTIVES: Medications often cause constipation and little data are available concerning treatment interventions. This study was designed to evaluate the safety and efficacy of polyethylene glycol (PEG) 3350 laxative (MiraLax) for relief of constipation from medicines associated with symptoms of constipation. METHODS: Study subjects were enrolled who met defined criteria for chronic constipation and were also taking medications that were associated with a reported side effect incidence of more than 3% constipation. Subjects were randomized into a double-blind, parallel, multicenter study where they received 17 g per day of PEG laxative or placebo for 28 days. The primary efficacy variable, "Treatment Success," was defined as relief of ROME II criteria for constipation over the last 7 days of the treatment period. Various secondary measures were also assessed. Daily bowel movement experience, patient perception of efficacy, and safety information were recorded in a diary. Laboratory testing was performed at baseline and at end of study for hematology and blood chemistry, including BUN, calcium, electrolytes, and TSH. RESULTS: One hundred patients were enrolled at 4 study centers. Successful treatment according to the primary efficacy variable was seen in 78.3% of PEG and 39.1% of placebo subjects (P < 0.001). Similar results were observed in a subgroup of 28 elderly subjects. Secondary measures of number of bowel movements, complete bowel movements, satisfactory bowel movements, straining at stool and stool consistency also showed statistically significant results in favor of PEG compared with placebo (P < or = 0.01) after the first week of treatment. There were no differences inpatient reported scores for gas, cramping, or bloating between PEG and placebo. No significant differences in laboratory findings or adverse events, including the gastrointestinal category, were observed. Diarrhea and flatulence occurred more frequently with PEG treatment, although they were not individually statistically different from placebo. Similar results were observed when these symptoms were analyzed for differences due to gender, race, or age. CONCLUSIONS: PEG laxative is safe and effective for use in treating constipation in patients taking constipating medications.