Bowel habits affect the quality of life (QOL) of patients with functional gastrointestinal disorders. This study evaluated the effects of reduced form coenzyme Q 10 (ubiquinol) intake on defecation frequency and stool form in patients with daily abdominal symptoms. This was a single-center, prospective, double-blind, randomized control study. Forty-one patients who had the daily symptom of constipation or diarrhea were randomly assigned at a 1:1 ratio to receive either ubiquinol (150 mg/day) or placebo for 12 weeks. Patients completed a daily diary to collect information regarding their numbers of defecations and stool forms according to the Bristol Stool Form (BSF) Scale for 7 days at baseline and 12 weeks. QOL was assessed using the 36-item short-form (SF-36) at baseline and 12 weeks. Twenty-one patients were assigned to the ubiquinol group, and 20 were assigned to the placebo group. At 12 weeks, the mean defecation frequency, compared to baseline, significantly decreased in the ubiquinol group (-0.1 times/day, P = .034) and increased in the placebo group (+0.3 times/day, P = .004). There was no significant change in the 12-week BSF Scale score of the ubiquinol group (+0.2, P = .123), whereas that of the placebo group was increased (+0.5, P < .001). The 12-week general health perception SF-36 score was significantly increased in the ubiquinol group (+3.5, P = .045), whereas there was no significant difference in that score in the placebo group (+1.2, P = .178). In conclusion, taking ubiquinol for 12 weeks decreased defecation frequencies and increased the QOL score, suggesting that ubiquinol may change the bowel habits and improve QOL in patients with abdominal distress.