Ondansetron is commonly used during pregnancy to manage severe nausea and vomiting when other treatments fail. Since you are in your fifth month, the baby's major organs are already formed, making the medication generally safe to continue under your doctor's supervision. Rest assured, ondansetron is not addictive, and your body does not become habituated to it. The return of vomiting when you stop simply means your underlying pregnancy-related nausea is still active, and the medication was doing its job. Please consult your obstetrician to discuss whether you should continue taking it or try gradually reducing the dose.