Antidiarrhoeal drugs are classified according to their mechanism of action:
Oral rehydration agents are used to re-balance fluid and electrolytes lost during a diarrhoeal episode. These contain defined quantities of salts and sugars to be taken with clean water.
Antibacterial agents can be used to treat diarrhea with a confirmed bacterial cause.
Antimotility agents, or antipropulsives, are used to slow intestinal transit. For example, the opioid analogue loperamide (sold as Imodium®) slows peristalsis and reduces overall stool mass (often combined with the anti-foaming agent simeticone in the brand Imodium Plus®). Diphenoxylate is another opioid analgesic used with atropine (Lomotil®) as an antimotility agent.
Antispasmodic agents are used to reduce the pain and cramping that can accompany diarrhea. For example mebeverine, sold as Colofac® in the UK.
Intestinal adsorbents such as purified naturally occurring clays (e.g., diosmectite and kaolin) and pectin, an indigestible carbohydrate derived from apples, can be used acutely to treat diarrhoea. They adsorb bacteria, bacterial toxins and fluid, decreasing stool liquidity and number. Intestinal adsorbents can impair the absorption of other drugs and so should not be taken within at least two hours of other orally administered drugs.
Colloidal bismuth compounds act as intestinal adsorbents of bacterial toxins but additionally have antibacterial and mucosal protective actions. Bismuth subsalicylate also rapidly dissociates, allowing the absorption of salicylate. Salicylic acid acts as a nonselective non-steroidal anti-inflammatory drug (NSAID), inhibiting cyclooxygenase-1 (COX-1). The resulting reduction in prostaglandin production lowers intestinal chloride secretion, reducing osmotic fluid retention in the lumen of the intestine. The antidiarrhoeal actions of bismuth occur in the lumen of the intestine, and bismuth is poorly absorbed. However, elimination is very slow once absorbed. So bismuth compounds should not be used frequently and should be avoided in patients with renal impairment to prevent accumulation in the body.
Lyophilisate of heat-killed Lactobacillus acidophilus can be used to treat mild bacterial diarrhoea. Lactobacillus acidophilus is found in the normal intestinal flora of people who eat fermented dairy products. Freeze-dried fragments of heat-inactivated Lactobacillus acidophilus adhere to the intestinal walls and, when administered in excess, can competitively exclude other microorganisms, preventing over-colonisation by bacteria causing diarrhoea. While it remains controversial whether live probiotic formulations can reliably achieve sufficient colonisation to competitively exclude pathogenic bacteria, lyophilisate of heat-killed Lactobacillus acidophilus can be administered at sufficiently high concentrations to have an antidiarrhoeal effect against bacterial diarrhoea.
Bile salt-binding resins, such as colestyramine, are used to treat colonic secretory diarrhoea caused by excess faecal bile salts. Bile salts are normally reabsorbed in the ileum. Disease (e.g., Crohn's disease) or surgical resection of the ileum can result in malabsorption of bile salts, causing diarrhoea due to an osmotic laxative-like effect of excess bile salts reaching the colon. Bile salt-binding resins prevent this effect. Bile salt-binding resins can impair the absorption of other drugs and so should not be given within at least two hours of other orally administered drugs.
Other antidiarrhoeal drugs include octreotide (a somatostatin mimetic) used for the treatment of diarrhoea associated with acute graft-versus-host disease, refractory diarrhoea associated with chemotherapy, and secretory diarrhoea associated with gastroenteropancreatic neuroendocrine tumours (e.g., gastrin-secreting or vasoactive intestinal peptide-secreting tumours).