Associated with adverse drug interactions - constipation

Red flags

Severe constipation with abdominal distension/vomiting - possible obstruction, faecal impaction with overflow diarrhoea
Autonomic dysfunction with severe constipation - consider antimuscarinic toxicity/ileus
Chronic opioid use presenting with acute abdomen - opioid-induced bowel dysfunction can mimic obstruction

Opioids (commonest, dose-related, does not develop tolerance to unlike other opioid effects)

Mu-receptor effect on gut - reduced motility, increased fluid absorption, sphincter tone increase

Anticholinergic drugs

Other common culprits

Iron supplements, calcium channel blockers (verapamil), 5-HT3 antagonists (ondansetron), aluminium-containing antacids
Clonidine, antiparkinsonian drugs

Severe/life-threatening

Vincristine and other vinca alkaloids - autonomic neuropathy causing severe constipation, ileus
Clozapine - potentially fatal ileus/bowel obstruction, under-recognised

Opioid dose and duration (constipation persists unlike other opioid tolerance effects)
Full anticholinergic drug burden
Clozapine use specifically - ask about bowel habit at every review
Fluid intake, mobility, dietary fibre

Abdominal distension, tenderness, bowel sounds
PR exam for impaction
Signs of obstruction (absent bowel sounds, peritonism) vs simple functional constipation

Prophylactic laxative with every regular opioid prescription - stimulant + softener combination (e.g., senna + docusate) rather than bulking agents alone
Peripherally-acting mu-opioid receptor antagonists (PAMORAs) - methylnaltrexone, naloxegol - for opioid-induced constipation refractory to standard laxatives, without reversing analgesia
Review and reduce anticholinergic burden where feasible
Clozapine: proactive bowel monitoring and laxative co-prescription is standard of care given fatal ileus risk
Faecal impaction - disimpaction (oral/rectal) before maintenance therapy

Unlike sedation/nausea, tolerance does not develop to opioid-induced constipation - laxatives are needed for the duration of opioid therapy, not just initiation
Clozapine-induced bowel obstruction/ileus is a leading cause of clozapine-related death and is easily missed if bowel habit isn't asked about proactively
Bulk-forming laxatives (psyllium) can worsen opioid-induced constipation if fluid intake is inadequate - avoid as first choice in this setting

"Every regular opioid script gets a laxative with it - no tolerance develops to this effect, so it can't be reviewed away later."
"Clozapine constipation gets asked about at every review - it's a known cause of preventable death."

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