Expansion
Bulk, osmotic, stimulant and softening agents
Mnemonic
Four classes, chosen by the problem:
- Bulk forming - ispaghula, methylcellulose. Need adequate fluid intake. First line in simple constipation, but avoid in opioid induced constipation and in obstruction
- Osmotic - macrogol, lactulose, phosphate enemas. Draw water into the bowel. Lactulose additionally reduces ammonia absorption, which is why it is used in hepatic encephalopathy
- Stimulant - senna, bisacodyl, sodium picosulfate, glycerol suppositories. Increase motility. Contraindicated in obstruction
- Softeners - docusate, arachis oil enema (avoid in peanut allergy)
“Bulk for the sluggish, osmotic for the hard, stimulant for the slow, softener for the impacted.”
Opioid induced constipation needs a stimulant, not bulk, because the problem is reduced motility rather than insufficient bulk. Every strong opioid prescription should be accompanied by one, since tolerance develops to almost every opioid effect except constipation.
Faecal impaction with overflow diarrhoea is the trap: it is treated with laxatives, not antidiarrhoeals.
Expansion
| Class | Examples | Mechanism and use |
|---|---|---|
| Bulk forming | Ispaghula, methylcellulose | Increase stool mass; need adequate fluid; avoid in opioid constipation and obstruction |
| Osmotic | Macrogol, lactulose, magnesium salts | Retain water in the lumen. Lactulose also acidifies the colon, hence its use in encephalopathy |
| Stimulant | Senna, bisacodyl, sodium picosulfate | Increase motility and secretion; act within 6 to 12 hours |
| Softeners | Docusate, arachis oil enema | Reduce surface tension |
| Peripheral opioid antagonists | Naloxegol, methylnaltrexone | For opioid-induced constipation without reversing analgesia |
| Prokinetics and secretagogues | Prucalopride, linaclotide | Chronic constipation and IBS |
Practical rules
- Always co-prescribe a laxative with an opioid, since tolerance to constipation never develops. A stimulant plus softener is the usual choice
- Avoid all laxatives in suspected bowel obstruction
- Bulk forming agents are unsuitable where fluid intake is poor or motility is impaired, since they can worsen impaction
- Faecal impaction requires rectal measures first, then oral agents
Chronic stimulant laxative use was traditionally said to damage the myenteric plexus, but the evidence for this is weak and should not prevent appropriate use.