What the label lists and what reports show
Alosetron side effects
Alosetron's FDA label: “Most common adverse reactions (incidence >2% and >placebo) in clinical studies were constipation, abdominal discomfort and pain, nausea, and gastrointestinal discomfort and pain.” FAERS holds 908 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is constipation (133 reports).
- Generic listed
- Boxed warning
Check it at the source: the alosetron FDA label on DailyMed (effective March 23, 2026, Legacy Pharma USA Inc.); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of LOTRONEX?LOTRONEX may cause serious side effects, including: See " What is the most important information I should know about LOTRONEX? " The most common side effects of LOTRONEX include: constipation stomach (abdominal) discomfort and pain nausea intestinal discomfort and pain These are not all the possible side effects of LOTRONEX. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
Source: FDA drug label, Legacy Pharma USA Inc., effective March 23, 2026 (SPL set c216e967-436f-475a-8e6a-2ad11adc63af), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsMost common adverse reactions (incidence >2% and >placebo) in clinical studies were constipation, abdominal discomfort and pain, nausea, and gastrointestinal discomfort and pain.
Table 1. Adverse Reactions Reported in ≥1% of Patients with Irritable Bowel Syndrome and More Frequently on LOTRONEX 1 mg Twice Daily Than Placebo
| Body System Adverse Reaction | Placebo (n = 2,363) | LOTRONEX 1 mg twice daily (n = 8,328) |
|---|---|---|
| Constipation | 6% | 29% |
| Abdominal discomfort and pain | 4% | 7% |
| Nausea | 5% | 6% |
| Gastrointestinal discomfort and pain | 3% | 5% |
| Abdominal distention | 1% | 2% |
| Regurgitation and reflux | 2% | 2% |
| Hemorrhoids | 1% | 2% |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
From the label: adverse reactions, continuedThe following adverse reactions are described in more detail in other sections of the label: Complications of constipation [see Boxed Warning, Warnings and Precautions ( 5.1 )] Ischemic colitis [see Boxed Warning, Warnings and Precautions ( 5.2 )] Most common adverse reactions (incidence >2% and >placebo) in clinical studies were constipation, abdominal discomfort and pain, nausea, and gastrointestinal discomfort and pain. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Legacy Pharma Inc. at 1-800-727-7151 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch. 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice. Patients with Irritable Bowel Syndrome: Table 1 summarizes adverse reactions from 22 repeat-dose studies in patients with IBS who were treated with 1 mg of LOTRONEX twice daily for 8 to 24 weeks. The adverse reactions in Table 1 were reported in 1% or more of patients who received LOTRONEX and occurred more frequently on LOTRONEX than on placebo. A statistically significant difference was observed for constipation in patients treated with LOTRONEX compared to placebo (p<0.0001). Table 1.
Source: FDA drug label, Legacy Pharma USA Inc., effective March 23, 2026 (SPL set c216e967-436f-475a-8e6a-2ad11adc63af), via openFDA; the full label on DailyMed
What do FAERS reports show?
908 reports list alosetron among the medicines taken, each a report of something that happened, not proof the medicine caused it; 47 are coded as a death and 211 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 20 reactions below, 8 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Alosetron: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions