12 CLINICAL PHARMACOLOGY12.1 Mechanism of ActionSodium picosulfate is hydrolyzed by colonic bacteria to form an active metabolite: bis-(p-hydroxy-phenyl)-pyridyl-2-methane,BHPM, which acts directly on the colonic mucosa to stimulate colonic peristalsis. Magnesium oxide and citric acid react to createmagnesium citrate in solution, which is an osmotic agent that causes water to be retained within the gastrointestinal tract.12.2 PharmacodynamicsThe stimulant laxative activity of sodium picosulfate together with the osmotic laxative activity of magnesium citrate produces apurgative effect which, when ingested with additional fluids, produces watery diarrhea.