There are two main groups of OTC stimulant laxative agents: diphenylmethane derivatives and anthraquinones.

Diphenylmethane derivatives

Compounds available are bisacodyl and sodium picosulfate.

• Bisacodyl acts mainly via stimulation of the mucosal nerve plexus of the large intestine and so takes rather longer to act (6-10 hours) than laxatives that act within the small intestine. It is minimally absorbed and appears to exert no systemic effects.
• Bisacodyl causes gastric irritation; there are therefore no oral liquid presentations and tablets are enteric coated. Tablets should be swallowed whole and should not be taken within one hour of an antacid, as this will lead to dissolution of the coating and release of the drug into the stomach.
• Sodium picosulfate becomes active following metabolism by colonic bacteria; it therefore has a relatively slow onset of action, usually acting within 10-14 hours. It is useful in young children.

Anthraquinones

• Anthraquinones are naturally occurring glycosides, used in the form of standardised plant extracts. They are hydrolysed by colonic bacteria to release derivatives of 1,8-dihydroxyanthraquinone, which are absorbed to a moderate degree but with little systemic consequence.
• They are believed to act through both direct stimulation of the intramural nerve plexus and interference with absorption of water across the intestinal wall.
• Senna is now virtually the only anthraquinone-containing natural product used as a laxative. Preparations are usually standardised to the content of sennoside B (between 7.5 mg and 20mg per tablet, or 7.5mg/5ml syrup).
• Anthraquinone-containing preparations derived from several plant sources were popular ingredients of laxative products for many years but have now mainly dropped out of use. Frangula extract, from Rhamnus frangula, is included with sterculia, a bulk-forming laxative, in one licensed product; cascara extract (from R. purshiana) and aloin (from Aloe vera and other aloe species) with senna in another.