Alosetron: severe IBS with diarrhea, dosing, and side effects
Last updated September 7, 2026.
**Alosetron is a prescription 5-HT3 receptor antagonist used for severe diarrhea-predominant irritable bowel syndrome in women.** It blocks serotonin type 3 receptors in the gut, which slows how fast stool moves through the colon and dials down the pain and urgency signals traveling from the bowel to the brain. The single most important safety point is in the boxed warning: alosetron can cause ischemic colitis and serious complications of constipation, some of them fatal. You stop the drug and call your doctor right away if you become constipated or develop new abdominal pain with blood in your stool.
What does it treat?
Alosetron is approved for women with severe diarrhea-predominant irritable bowel syndrome (IBS-D) who have had symptoms for six months or longer, who have had other causes ruled out by testing, and who did not get adequate relief from conventional treatments such as fiber changes, antispasmodics, or loperamide. The label defines severe as diarrhea plus at least one of the following: frequent and severe abdominal pain or discomfort, frequent bowel urgency or fecal incontinence, or disability and restriction of daily activities. It is not approved for men, because effectiveness was not established in male patients. It is not for constipation-predominant or mixed IBS, and it is not used for inflammatory bowel disease, infectious diarrhea, or ordinary short-term diarrhea. It is sold as the brand Lotronex and as generic alosetron hydrochloride tablets, and it does not come in combination products.
How do you take it?
Alosetron comes as 0.5 mg and 1 mg oral tablets. The label starting dose for adults is 0.5 mg twice a day for four weeks. If you tolerate it but your symptoms are still not controlled, your doctor may raise it to 1 mg twice a day, which is the daily ceiling of 2 mg. If symptoms are still not controlled after four weeks at 1 mg twice a day, the drug is stopped rather than pushed higher. You can take it with or without food. If you miss a dose, skip it and take the next dose at the usual time; do not double up. Stop taking alosetron and call your doctor immediately if you become constipated or develop new or worsening abdominal pain or blood in your stool, and do not restart it on your own.
Side effects to know
- Common: The most common effect is constipation, which affects roughly one in four people in trials, along with abdominal pain or discomfort, nausea, and gassiness.
- Serious: Stop alosetron and get medical help right away for signs of ischemic colitis, meaning new or worsening abdominal pain with bloody diarrhea or rectal bleeding, and for serious constipation complications such as no bowel movement for several days, a hard swollen belly, or vomiting, which can signal obstruction, perforation, or toxic megacolon. Also stop and seek care for signs of an allergic reaction such as hives, facial or throat swelling, or trouble breathing.
Who should not take it?
Do not take alosetron if you have a history of chronic or severe constipation, a bowel obstruction, stricture, toxic megacolon, perforation, or adhesions, or if you have had ischemic colitis, poor blood flow to the intestines, blood clots in veins, or a clotting disorder. It is also contraindicated in Crohn disease, ulcerative colitis, diverticulitis, severe liver disease, or a past allergic reaction to alosetron. Do not combine it with fluvoxamine; that pairing is contraindicated because fluvoxamine raises alosetron levels sharply. Use caution with other drugs that block the CYP1A2 enzyme, such as ciprofloxacin, and tell your doctor about all medicines that slow the bowel, including opioids, anticholinergics, and antidiarrheals, since they raise the constipation risk. There is no adequate human data on alosetron in pregnancy, so tell your doctor if you are pregnant, planning a pregnancy, or breastfeeding, since it is not known whether alosetron passes into human milk and the decision is made case by case. The label does not set an alcohol rule, but alcohol commonly triggers IBS symptoms, so keep intake low and tell your doctor what you drink.
When is it an emergency?
Call 911 if you collapse, have trouble breathing, or have swelling of the face, lips, or throat. Go to an emergency room now for severe abdominal pain, bloody diarrhea or rectal bleeding, a rigid or badly swollen belly, repeated vomiting with no bowel movement or gas, fever with abdominal pain, or lightheadedness with a racing heart, since these point to ischemic colitis or a bowel complication that needs imaging and in-person care within hours. For a suspected overdose or if a child swallows the tablets, call Poison Control at 1-800-222-1222 right away, even if the person looks fine.
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Common questions
What is the maximum dose of alosetron?
The ceiling is 1 mg twice a day, which is 2 mg total per day. Everyone starts at 0.5 mg twice a day for four weeks, and only moves up if that dose is tolerated but symptoms are still not controlled. If four weeks at 1 mg twice a day does not control your symptoms, the label says to stop the drug rather than increase it. Taking more than 2 mg a day does not add benefit and raises the risk of constipation and its complications.
Can I drink alcohol while taking alosetron?
The label does not list a direct interaction between alosetron and alcohol. The practical problem is that alcohol is a common trigger for IBS symptoms and can worsen diarrhea, cramping, and urgency, which works against what alosetron is doing. Alcohol also strains the liver, and alosetron is cleared by the liver and is not used in severe liver disease. Tell your doctor how much you drink so your dose and liver status can be reviewed.
Is alosetron safe during pregnancy or breastfeeding?
There is not enough human data to say alosetron is safe in pregnancy, so it is not a drug to continue on your own if you become pregnant. Tell your doctor as soon as you are pregnant or planning a pregnancy so the decision can be made against how severe your IBS is. It is not known whether alosetron passes into human breast milk, though it appears in the milk of rats. Discuss it with your doctor before breastfeeding rather than stopping or starting on your own.
Do I take alosetron with food?
You can take it with or without food, so pick whichever is easier to remember and stay consistent. Doses are spaced roughly twelve hours apart, for example morning and evening. Food does not meaningfully change how well it works. If a dose upsets your stomach, taking it with a small meal is reasonable.
What are the signs I have taken too much or need to stop?
The warning signs that matter most are constipation, new or worsening abdominal pain, and blood in your stool or rectal bleeding. Any of those means stop alosetron and call your doctor the same day, no matter what dose you are on. A hard swollen belly, vomiting with no bowel movement or passed gas, or fever with abdominal pain means emergency care now. For a suspected overdose or an accidental swallow by a child, call Poison Control at 1-800-222-1222 right away.
What medicines interact with alosetron?
Fluvoxamine is contraindicated because it strongly blocks the CYP1A2 enzyme that clears alosetron and drives blood levels up. Other CYP1A2 blockers such as ciprofloxacin need caution and a doctor's review. Anything that slows the bowel stacks the constipation risk, including opioid pain medicines, loperamide, anticholinergics, and some antidepressants and bladder drugs. Bring your full list, including over the counter products and supplements, before starting.