Are there any scientific studies evaluating the efficacy of colon hydrotherapy (colonics)?

No: No scientific studies have ever shown a benefit to colonics. In fact, what many people do not realize is that the colon lining gets nourished from free fatty acids in the stool. Basically the poop in the colon nourishes the colon! any therapies geared towards removing waste are potentially damaging to the colon!

Colonics: There is some research on the efficacy & safety of colon irrigation/ hydrotherapy. "Clinical effects of colonic cleansing for general health promotion: a systematic review"(Acosta RD & Cash BD) is one such study. Findings: “ The investigators concluded that there are no methodologically rigorous controlled trials of colonic cleansing to support the practice for general health promotion.”