What the label lists and what reports show
Ondansetron side effects
Ondansetron's FDA label: “The most common adverse reactions in adults for the: • prevention of chemotherapy-induced (≥5%) are: headache, malaise/fatigue, constipation, diarrhea.” FAERS holds 121,842 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 fatigue (10,275 reports).
- Generic listed
Check it at the source: the ondansetron FDA label on DailyMed (effective January 8, 2026, Glenmark Pharmaceuticals Inc., USA); the FDA FAERS public dashboard.
What does the label list?
From the label: adverse reactionsThe most common adverse reactions in adults for the: • prevention of chemotherapy-induced (≥5%) are: headache, malaise/fatigue, constipation, diarrhea.
Table 4: Most Common Adverse Reactions in Adults 1 for the Prevention of Postoperative Nausea and Vomiting
| Adverse Reaction | Ondansetron 16 mg as a Single Dose (n = 550) | Placebo (n = 531) |
|---|---|---|
| Headache | 49 (9%) | 27 (5%) |
| Hypoxia | 49 (9%) | 35 (7%) |
| Pyrexia | 45 (8%) | 34 (6%) |
| Dizziness | 36 (7%) | 34 (6%) |
| Gynecological disorder | 36 (7%) | 33 (6%) |
| Anxiety/Agitation | 33 (6%) | 29 (5%) |
| Urinary retention | 28 (5%) | 18 (3%) |
| Pruritus | 27 (5%) | 20 (4%) |
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 clinically significant adverse reactions are described elsewhere in the labeling: • Hypersensitivity Reactions [see Warnings and Precautions ( 5.1 )] • QT Prolongation [see Warnings and Precautions ( 5.2 )] • Serotonin Syndrome [see Warnings and Precautions ( 5.3 )] • Myocardial Ischemia [see Warnings and Precautions ( 5.4 )] • Masking of Progressive Ileus and Gastric Distension [see Warnings and Precautions ( 5.5 )] The most common adverse reactions in adults for the: • prevention of chemotherapy-induced (≥5%) are: headache, malaise/fatigue, constipation, diarrhea. ( 6.1 ) • prevention of radiation-induced nausea and vomiting (≥2%) are: headache, constipation, and diarrhea. ( 6.1 ) • prevention of postoperative nausea and vomiting (≥9%) are: headache and hypoxia. ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Glenmark Pharmaceuticals Inc., USA at 1 (888) 721-7115 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 with rates in the clinical trials of another drug and may not reflect the rates observed in practice. The following adverse reactions have been reported in clinical trials of patients treated with ondansetron, the active ingredient of ondansetron. …
Source: FDA drug label, Glenmark Pharmaceuticals Inc., USA, effective January 8, 2026 (SPL set eee5217a-eba8-463a-812d-bf3c66f0934f), via openFDA; the full label on DailyMed
What do FAERS reports show?
121,842 reports list ondansetron among the medicines taken, each a report of something that happened, not proof the medicine caused it; 15,159 are coded as a death and 53,916 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, 9 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?
- Ondansetron: 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