[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Drugs for irritable bowel syndrome](https://durgical.com/en/pharmacology/classes/drugs-for-irritable-bowel-syndrome/) › Alosetron

# Alosetron

## Drugs for irritable bowel syndrome?

Drugs for irritable bowel syndrome

A 5-HT3 blocker for women with severe IBS with diarrhoea that has not responded to other care. It can cause ischaemic colitis and severe constipation.

Updated Nov 1, 2026

## Quick facts

**Group**
[Drugs for irritable bowel syndrome](https://durgical.com/en/pharmacology/classes/drugs-for-irritable-bowel-syndrome/)

**Target**
5-HT3 receptor (ligand-gated cation channel) on enteric neurons

**Key effects**
Constipation · Abdominal discomfort and pain · Nausea

**Monitor**
Bowel habit · Rectal bleeding

**ATC**
A03AE01

No doses: the clinician sets them by the current guideline.

On this page

- The basics Mechanism of action
- Indications
- Adverse effects
- Pharmacokinetics
- Memory aid

- Safety Contraindications
- Key interactions
- What is monitored
- Administration and safety
- Pregnancy and breastfeeding
- Nursing care

- Study Test yourself

- Connections Connections

## Mechanism of action

⇅**5-HT3 receptor (ligand-gated cation channel) on enteric neurons**Alosetron

ⓘ The figure shows alosetron bound to the mouse 5-HT3A receptor, which closely resembles the human channel.

**Target in 3D:** 5-HT3 receptor (ligand-gated cation channel) on enteric neurons. PDB structure 6W1J (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/6W1J) · Drag to rotate · pinch to zoom

Alosetron is a potent, selective 5-HT3 receptor antagonist. 5-HT3 receptors are cation channels on enteric neurons that, when activated by serotonin, affect visceral pain, colonic transit and secretion. Blocking them modulates the enteric nervous system: transit slows, urgency and pain ease. Too much slowing explains constipation, and reduced colonic blood flow in some patients explains ischaemic colitis.

- Alosetron blocks 5-HT3 channels on enteric neurons
- Less neuronal depolarisation in the gut wall
- Colonic transit slows; urgency and pain ease

## Indications

- Severe diarrhoea-predominant irritable bowel syndrome in women, chronic, with other causes excluded and no response to conventional therapy [Irritable bowel syndrome →](https://durgical.com/en/conditions/irritable-bowel-syndrome/)

## Adverse effects

### Common

- Constipation, the most common effect
- Abdominal discomfort and pain
- Nausea
- Flatulence

### Serious (rare, important)

- Ischaemic colitis: rectal bleeding, bloody diarrhoea, new or worse abdominal pain (boxed warning)
- Serious complications of constipation: obstruction, ileus, impaction, toxic megacolon, perforation

## Contraindications

- Starting in a patient who is constipated
- History of chronic or severe constipation, intestinal obstruction, stricture, toxic megacolon, perforation or adhesions
- Ischaemic colitis, impaired intestinal circulation, thrombophlebitis or a hypercoagulable state
- Crohn's disease, ulcerative colitis, diverticulitis
- Severe hepatic impairment; use with fluvoxamine

## Key interactions

**Fluvoxamine (strong CYP1A2 inhibitor)**
Much higher alosetron levels and longer half-life; contraindicated

**Moderate CYP1A2 inhibitors (fluoroquinolones, cimetidine)**
Likely higher levels; avoided unless needed

**Strong CYP3A4 inhibitors (ketoconazole, clarithromycin, protease inhibitors)**
Higher exposure; used with caution

**Drugs that slow the bowel (opioids, anticholinergics)**
Higher risk of serious constipation

## What is monitored

- Bowel habit: constipation means stopping at once
- Rectal bleeding, bloody diarrhoea or new abdominal pain
- Symptom control after the first weeks

## Administration and safety

- Oral tablet; started at a low dose and stopped if symptoms are not controlled after a few weeks at the full dose.
- Not restarted after ischaemic colitis.
- Older, frail patients and those on bowel-slowing drugs are at higher risk of constipation complications.

## Pregnancy and breastfeeding

Human data are insufficient to define a risk; animal studies showed no developmental harm.

## Nursing care

- Ask about constipation, past bowel obstruction, IBD, diverticulitis, clotting disorders and liver disease before starting.
- Check the medication list for fluvoxamine and other CYP1A2 inhibitors.
- Teach the patient to stop the drug and call at once for constipation, rectal bleeding, bloody diarrhoea or worsening pain.
- Give and explain the medication guide.

## Pharmacokinetics

**Absorption**
Rapidly absorbed orally; food lowers absorption somewhat

**Metabolism**
Mainly hepatic CYP1A2, with minor CYP3A4 and CYP2C9

**Half-life**
Short, about one and a half hours

## Memory aid

«Alo-SETRON stops the run, but watch the colon»: constipation or blood means stop now.

## Test yourself

Think of the answer, then tap the question to see it.

## Which patients is alosetron limited to?

Women with severe, chronic diarrhoea-predominant IBS who did not respond to conventional therapy.

## A patient on alosetron has rectal bleeding. What is suspected and what is done?

Ischaemic colitis; the drug is stopped immediately and not restarted.

## Which antidepressant is contraindicated with alosetron and why?

Fluvoxamine, a strong CYP1A2 inhibitor that greatly raises alosetron levels.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/alosetron/)

[More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology)

## Connections

## Related drugs 3

- [Eluxadoline](https://durgical.com/en/pharmacology/eluxadoline/)
- [Ondansetron](https://durgical.com/en/pharmacology/ondansetron/)
- [Loperamide](https://durgical.com/en/pharmacology/loperamide/)

## Related conditions 1

- [Irritable bowel syndrome](https://durgical.com/en/conditions/irritable-bowel-syndrome/)

[← Back to the class: Drugs for irritable bowel syndrome](https://durgical.com/en/pharmacology/classes/drugs-for-irritable-bowel-syndrome/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - ALOSETRON HYDROCHLORIDE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d62b2c5a-1693-4e67-96d5-197e1fdc8694) (DailyMed (U.S. National Library of Medicine), 2025); [Alosetron: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a601230.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=A03AE01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); [RCSB PDB - 6W1J: Cryo-EM structure of 5HT3A receptor in presence of Alosetron](https://www.rcsb.org/structure/6W1J) (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/alosetron/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/alosetron/