Senna is the stronger laxative compared to Movicol (polyethylene glycol/PEG)
Senna is a stimulant laxative that directly stimulates colonic peristalsis and secretion, making it more potent than PEG, which works as an osmotic agent. However, this greater strength comes with more side effects and is best reserved for short-term or rescue use, while PEG is better tolerated for regular, ongoing management of constipation.
Mechanism and Strength Comparison
Senna (sennosides) works by being metabolized by gut bacteria into active compounds (rheinanthrone and rhein) that stimulate prostaglandin E2 production and chloride secretion, directly triggering colonic contractions and water secretion 1. This direct stimulation of the colon makes it a more forceful laxative.
Movicol (PEG) functions as an osmotic laxative, drawing water into the intestinal lumen through osmotic pressure. This is a gentler, more physiologic mechanism that doesn't directly stimulate the bowel 2.
Clinical Evidence and Recommendations
According to the 2023 AGA-ACG guidelines 1:
- Stimulant laxatives (including senna) receive a conditional recommendation with LOW certainty evidence
- The guidelines note that senna increased complete spontaneous bowel movements by 7.6 per week compared to placebo (MD 7.60,95% CI 5.90–9.30) 1
- 83% of patients in trials reduced their senna dose due to side effects, indicating its potency often exceeds what patients can tolerate 1
Side Effect Profile Reflects Potency
Senna's greater strength manifests in more frequent adverse effects:
- Abdominal pain and cramping are common, especially at higher doses 1
- The trial dose (1g daily) was higher than typically used in practice (8-9mg tablets), requiring dose reduction in most patients 1
PEG causes primarily:
Clinical Application Algorithm
For immediate relief or rescue therapy:
- Use senna for its stronger, faster effect
- Start with low dose (8-9mg) and increase only if needed
- Limit to short-term use (≤4 weeks) or occasional rescue 1
For ongoing constipation management:
- Use PEG (Movicol) as first-line for better tolerability
- Can be used long-term without the same concerns as stimulant laxatives 2
If PEG fails:
- Add senna as rescue therapy or switch temporarily
- Consider prescription agents (linaclotide, plecanatide, prucalopride) for refractory cases 1
Important Caveats
- Antibiotics can reduce senna's effectiveness by affecting gut bacteria needed to convert sennosides to active metabolites 1
- Both agents are contraindicated in bowel obstruction, ileus, or severe dehydration 1
- The "stronger" effect of senna doesn't necessarily mean better outcomes—the increased side effects may limit adherence and quality of life
In pediatric populations, recent evidence suggests PEG is preferred over senna (RR 1.35,95% CI 1.05-1.74) with less pain (RR 0.62,95% CI 0.44-0.87) 3, further supporting that senna's potency comes at the cost of tolerability.