Mechanisms of Action: Bisacodyl and Lactulose
Bisacodyl: Stimulant Laxative
Bisacodyl works by directly irritating sensory nerve endings in the colonic mucosa, which stimulates colonic motility and reduces colonic water absorption, resulting in faster transit and softer stools. 1
Detailed Mechanism
Requires activation: Bisacodyl must be converted to its active metabolite bis-(p-hydroxyphenyl)-pyridyl-2-methane (BHPM) in the gut before it can exert its laxative effect 2
Dual action: Acts locally in the large bowel through both prokinetic (motility-enhancing) and secretory mechanisms 2
Direct colonic effects: Enhances motility, reduces transit time, and increases water content of stool by irritating luminal sensory nerve endings 1, 2
Suppository formulation: When used rectally, bisacodyl acts as both a stool softener and stimulant for rectal motility 1
Clinical Onset and Duration
Oral administration: Best taken in the evening or at bedtime to produce a normal stool the following morning 1
Expected response: Clinical improvement typically occurs within 24-48 hours 3
Lactulose: Osmotic Laxative
Lactulose is a synthetic disaccharide that draws water into the intestinal lumen through osmotic action, hydrating and softening stool while promoting peristalsis. 1, 3
Detailed Mechanism
Non-absorbable sugar: Lactulose is not absorbed by the small bowel, allowing it to reach the colon intact where it exerts its osmotic effect 1
Water retention: Draws water into the gut lumen, thereby hydrating the stool and increasing its volume 1
Colonic fermentation: Bacterial metabolism of lactulose in the colon contributes to its osmotic effect and promotes peristalsis 3
Delayed onset: Has a latency of 2-3 days before onset of effect when first initiated 1
Clinical Characteristics
Dosing flexibility: FDA-approved dosing ranges from 10-20 g (15-30 mL) daily, titrated up to 40 g (60 mL) daily based on response 4, 3
Goal of therapy: Produce 2-3 soft, non-forced bowel movements daily 3, 5
Common side effects: Bloating and flatulence occur in approximately 20% of patients due to colonic fermentation, and are dose-dependent 3, 6
Key Mechanistic Differences
Site of action: Bisacodyl acts primarily through neural stimulation of the colon, while lactulose works through physical osmotic forces 1
Speed of onset: Bisacodyl typically works faster (within 6-12 hours orally, 15-60 minutes rectally) compared to lactulose's 24-48 hour onset 1, 3
Predictability: Lactulose provides more predictable, gentle softening, while bisacodyl's stimulant action can cause more variable responses with cramping 6, 5