Introduction and hypothesis: Constipation is defined as infrequent stools/difficultly passing stools, or the sensation of incomplete emptying. The prevalence of constipation is estimated at 14% worldwide. Caffeine is believed to increase gastric motility in the colon, causing a laxative effect. The results of the studies in the literature are conflicting. The aim of this review is to assess the effectiveness of caffeine in reducing constipation in adults.

Methods: We searched the Cochrane Library, AMED, MEDLINE, EMBASE, Web of Science and CINAHL. Search terms included "constipation", "faecal impaction" and "caffeine" in different forms. The screening of the titles was carried out by two independent reviewers for trials that assessed the effect of caffeine on constipation. We assessed the quality of the studies included and the data collated. ROBINS-I and risk-of-bias tools were employed to assess for risk of bias in the studies included.

Results: Eight studies were included in the final review. The studies assessed caffeine in the forms of coffee, tea, energy drinks, cocoa and kombucha with different doses. Three studies were at a low risk of bias, 3 were at a moderate risk of bias and the randomised controlled trial (RCT) had some concerns regarding blinding. Four studies concluded that caffeine may be effective in reducing constipation and 4 studies showed no improvement or an increase in constipation. Meta-analysis was not done as the studies were too dissimilar in interventions and outcome assessments.

Conclusion: The results of the studies are conflicting. Further rigorously designed and conducted RCTs need to be carried out to substantiate the effectiveness of caffeine on reducing constipation in adults.