What are the side effects of Alosetron?
Last updated September 7, 2026.
**The most common side effect of alosetron is constipation, and its most serious risks are ischemic colitis and complications of severe constipation such as bowel obstruction or perforation.** Your risk depends on your dose, how long you take the drug, and your bowel history, including any past constipation, Crohn disease, ulcerative colitis, diverticulitis, or clotting problems. Alosetron is approved only for women with severe diarrhea-predominant IBS who did not improve with other treatment, so it is not used for mild symptoms or in men. Most people start at the lowest dose twice daily, and the dose goes up only if diarrhea is still uncontrolled after four weeks. Milder effects such as abdominal pain, nausea, gas, and headache are more common than the serious ones. Drinking enough fluid and reporting any slowing of your bowels early are the two habits that keep a side effect from becoming an emergency.
What to do
- Track your bowel movements: Note any day you do not pass stool, since constipation is the earliest signal that the dose is too high for you.
- Stop for red flags: Stop the drug and call your doctor the same day for constipation, new or worsening abdominal pain, or blood in your stool.
- Start at the lowest dose: Most people begin at 0.5 mg twice daily and only move up if diarrhea is still uncontrolled after four weeks.
- Share your full gut history: Tell your prescriber about past constipation, Crohn disease, ulcerative colitis, diverticulitis, bowel surgery, or blood clots before starting.
- Check drug interactions: Alosetron should not be taken with fluvoxamine, and other medicines such as ciprofloxacin can raise its level in your body.
- Set a stop point: If your symptoms are not controlled after four weeks at the higher dose, alosetron is usually stopped rather than continued.
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