Abstract Stop Fitan® [manufactured by Demar Snc, Cesena (FC), Italy, on behalf of Geosilva, Cesena] is a dietary supplement proposed as a co-adjuvant in the therapy of diarrhea. It is based on the bioactive purified natural extract of chestnut (Castanea sativa) wood and Saccharomyces boulardii, a nonpathogenic yeast strain that has been used for treatment and prevention of diarrhea. The effects of Stop Fitan and the purified natural extract of chestnut wood were assessed in vitro using guinea pig ileum and proximal colon tissues. In order to explain their effects on intestinal smooth muscle contraction, a series of pathways implicated in intestinal motility have been investigated. In particular, the antispasmodic effect of natural extract of chestnut wood, containing hydrolyzable tannins, was tested against the spasmodic effects induced by carbachol, histamine, potassium chloride, and barium chloride in guinea pig ileum and by carbachol or serotonin in guinea pig proximal colon. The data show that natural extract of chestnut wood exerts spasmolytic effects in ileum and proximal colon, by a mechanism perhaps involving unspecific cellular pathways. These findings, taken together with the antibacterial, antiviral, and antispasmodic properties of tannins, suggest that the combination of tannins and S. boulardii may be relevant to treat diarrhea by Stop Fitan.