How do stimulant laxatives act?

The effect of stimulant laxatives is probably produced mainly by stimulation of the intestinal mucosa to secrete water and electrolytes. This is achieved through one or both of two possible mechanisms:

• Inhibition of the sodium pump – the enzyme sodium-potassium adenosine triphosphatase (Na+/K+ ATPase) – which prevents sodium transport across the intestinal wall, leading to the accumulation of water and electrolytes in the gut lumen
• Increased production of fluid in the intestine through the action of the laxative on cyclic adenosine monophosphate (cAMP) and prostaglandins, which promote active secretory processes in the intestinal mucosa.

Stimulant laxatives may also cause direct damage to mucosal cells,thereby increasing the permeability of these cells and allowing fluid to leak out.

Onset of action varies according to site of action, which may be the small intestine, large intestine, or both, but they normally work within 4-12 hours of administration. For this reason, doses are usually taken at bedtime. Suppository presentations (eg, bisacodyl) produce much faster results, usually within an hour.