Linaclotide
Linaclotide (Linzess), guanylate cyclase-C agonist
Written by Aaron CuhaReviewed Sep 2026
Also known as: Linzess, Constella, MD-1100
A 14-amino-acid peptide taken orally that works without being absorbed, acting entirely on receptors in the gut lining. Approved on randomised phase 3 trials for constipation.
Overview
Linaclotide is the most interesting design on this page and the reason is counterintuitive: it is an oral peptide that works precisely because it is not absorbed.
Oral peptides are normally a pharmaceutical problem, because the gut destroys them and almost nothing reaches the bloodstream. Linaclotide turns that into the mechanism. Its target, guanylate cyclase-C, sits on the luminal surface of intestinal epithelial cells, so the drug only needs to survive long enough to reach the receptor it is already sitting next to. Systemic exposure is minimal by design.
It is approved for irritable bowel syndrome with constipation and for chronic idiopathic constipation, on randomised phase 3 evidence.
Mechanism of action
A 14-amino-acid peptide agonist at guanylate cyclase-C on the apical surface of intestinal enterocytes, mimicking the endogenous peptides guanylin and uroguanylin. Activation raises intracellular and extracellular cyclic GMP, which activates the cystic fibrosis transmembrane conductance regulator chloride channel, driving chloride and bicarbonate into the lumen with sodium and water following. That increases intestinal fluid and accelerates transit. The extracellular cyclic GMP is also thought to reduce visceral pain by acting on submucosal afferent nerves, which is the proposed explanation for the pain benefit in irritable bowel syndrome as distinct from the laxative effect.
Human evidence
Multiple randomised phase 3 trials supporting two adult indications and a paediatric one, with a well-characterised and predictable adverse effect profile.
- Randomised phase 3 evidence in irritable bowel syndrome with constipation, including multiregional trials.
- Separately approved for chronic idiopathic constipation in adults and functional constipation in children.
- Diarrhoea is the dominant adverse effect and follows directly from the mechanism, being the intended effect in excess rather than an unrelated toxicity.
- Systemic exposure is minimal, which limits the potential for drug interactions and systemic adverse effects.
- A boxed warning contraindicates use in children under two years because of serious dehydration risk.
What this does not tell you: The endpoints in this field are symptom composites, which are inherently subjective and subject to substantial placebo response, so effect sizes look modest relative to the placebo arm. Nothing here concerns ageing, longevity or any use outside the approved gastrointestinal indications. The visceral pain mechanism through extracellular cyclic GMP is a plausible explanation rather than an established one.
Reading the research record
The design lesson on this page is worth extracting because it applies across the site. Oral peptide delivery is generally treated as the hard problem to solve, and enormous effort goes into protecting peptides from the gut so they can reach the bloodstream. Linaclotide sidesteps that entirely by having its target in the gut lumen. Minimal absorption is a feature.
That is relevant to anyone evaluating oral peptide products with systemic claims. If a peptide is meant to act on muscle, brain or joint tissue, it has to survive digestion and cross into circulation, and almost none do. If its target is the intestinal surface, that requirement disappears. The question to ask of any oral peptide is where its receptor is.
The evidence, charted
Fig. 1 · evidence composition
1of 1 citation (100%) is in people
Every citation cited here is Human work; no other study type is cited on this page. This count is of our own citation list and understates any literature larger than the sources we cite.
Fig. 2 · evidence over time
Every citation here was published in 2018.
Too few distinct publication years on this page to plot as a timeline. The newest citation on file is from 2018, more than five years ago; the published record may have gone quiet.
Fig. 3 · legal status at a glance
US
Approved
UK
Approved
AU
Approved
CA
Approved
Approved in all four jurisdictions shown. A jurisdiction's classification is a regulatory fact, not a verdict on the science; see Legal status below for the full text and any notes.
Key studies & citations
- Human2018
Linaclotide in irritable bowel syndrome with constipation: a phase 3 randomized trial in China and other regions
A randomised phase 3 trial of linaclotide in irritable bowel syndrome with constipation. One of the registrational trials supporting approval in this indication.
Journal of Gastroenterology and Hepatology
Frequently asked questions
How can an oral peptide work when the gut destroys peptides?
Because its target is in the gut. Linaclotide acts on a receptor on the luminal surface of intestinal cells, so it never needs to enter the bloodstream. Systemic absorption is minimal by design, which is the opposite of the problem most oral peptides have.
Does that mean other oral peptides work too?
No, and this is the useful inference. A peptide whose target sits on the intestinal surface has an easy path. A peptide meant to act on muscle, brain or joints must survive digestion and cross into circulation, and very few do. Ask where the receptor is before believing an oral claim.
What is the main side effect?
Diarrhoea, which is the intended mechanism in excess rather than an unrelated toxicity. It is the most common reason people stop, and there is a boxed warning against use in children under two because of serious dehydration risk.