Ondansetron is a medication best known as a serotonin (5-HT3) receptor antagonist used to prevent nausea and vomiting from chemotherapy, surgery, or illness. In opioid withdrawal, ondansetron can help reduce nausea, vomiting, and sometimes diarrhea, which are common and distressing symptoms. It does not act on opioid receptors, so it does not relieve cravings or the core withdrawal syndrome, but it provides supportive symptom relief. Because it is generally well tolerated and non-sedating, ondansetron is often used alongside other treatments such as buprenorphine, methadone, clonidine, or lofexidine to make detoxification more manageable.