Diarrhea is the most common side effect of Linzess (linaclotide), a prescription capsule for chronic constipation and irritable bowel syndrome with constipation, and it is a direct result of how the drug works. Linaclotide is a small peptide that mimics the gut’s own fluid-releasing hormones, pulling salt and water into the intestine to soften stool and speed it along. Sometimes that overshoots. In the main adult trials, about one in five people on the drug reported diarrhea, usually in the first two weeks. Mild loose stools are common; severe or dehydrating diarrhea means stopping and calling your prescriber.
A drug modeled on the gut’s own peptides
The Linzess prescribing information describes linaclotide as a 14-amino-acid peptide that is structurally related to human guanylin and uroguanylin. The UniProt record for guanylin describes it as a natural activator of an enzyme in the intestinal lining called guanylate cyclase-C, and the UniProt record for uroguanylin says it may be a potent regulator of intestinal fluid and salt transport.
According to the label, linaclotide acts locally on the inner surface of the intestine. Switching on guanylate cyclase-C raises a messenger molecule called cyclic GMP, which drives chloride and bicarbonate into the gut, mainly through the same CFTR channel involved in cystic fibrosis. Water follows, the result is more intestinal fluid and faster transit, and in animal studies the drug also reduced the activity of pain-sensing nerves. Very little of the drug is absorbed into the bloodstream, the label notes.
How common diarrhea is, and when it starts
In the two pivotal placebo-controlled trials in adults with IBS with constipation, 20% of people taking Linzess reported diarrhea compared with 3% on placebo. Severe diarrhea was reported in 2% versus less than 1%, and 5% stopped the drug because of it. The label says most cases began within the first two weeks of treatment. Abdominal pain, gas, bloating and headache were the other common side effects.
Timing around meals matters too. The label reports that taking the capsule right after a high-fat breakfast led to looser and more frequent stools than taking it on an empty stomach, which is how it was given in the trials. Follow the timing your prescription label gives, and if diarrhea is a problem, ask whether timing or a different strength could help rather than adjusting on your own.
Warning signs that need a call
MedlinePlus says to stop taking linaclotide and call your doctor right away or get emergency care for:
- Unusual or severe stomach pain.
- Bright red, or black and tarry, stools.
- Dizziness or fainting.
- Extreme thirst, dry mouth or skin, sunken eyes or a fast heartbeat, which can signal dehydration.
- Hives, rash, itching, swelling of the lips, tongue or throat, or trouble breathing.
The label’s boxed warning concerns young children: Linzess must not be given to children under 2, because in newborn mice a single dose caused deaths from dehydration. Keep the capsules out of reach of small children. The drug is also not for anyone with a known or suspected bowel blockage.
Where it fits among constipation treatments
The NIDDK describes linaclotide and the related drug plecanatide as prescription options when self-care has not worked, alongside lubiprostone and prucalopride. A 2023 joint guideline from the American Gastroenterological Association and American College of Gastroenterology on chronic idiopathic constipation made strong recommendations for linaclotide and plecanatide, as well as for polyethylene glycol, sodium picosulfate and prucalopride, and conditional ones for fiber, lactulose, senna, magnesium oxide and lubiprostone. The panel urged shared decisions that weigh patient preference, cost and availability.
MedlinePlus notes that it can take a week or longer to feel the full effect, and that symptoms may return about a week after stopping. It controls symptoms but does not cure the condition, so talk with your prescriber before stopping.
By The Everyday Brief · AI-assisted editorial. Drafted with AI research assistance; claims checked against the linked sources (fetched 2026-10-05 PT). Not medical advice.
Sources
- DailyMed: Linzess (linaclotide) prescribing information
- MedlinePlus (U.S. National Library of Medicine): Linaclotide
- NIDDK: Treatment for constipation
- Chang L, et al. AGA-ACG clinical practice guideline: pharmacological management of chronic idiopathic constipation. Gastroenterology, 2023.
- UniProt: Guanylin (Q02747)
- UniProt: Guanylate cyclase activator 2B, uroguanylin (Q16661)