What medicines are like zofran (ondansetron) meaning prevent vomiting not only from chemo but also from gastroenteritis but prescription not needed in order to buy?

Not many good ones.: Most good anti-vomiting drugs require a prescription in the U.S. Zofran definitely requires one. For diarrhea and stomach upset from gastroenteritis, try Pepto Bismol or Kaopectate. Some of the antihistamines like Benadryl (diphenhydramine) or Dramamine can also help with nausea. For severe nausea and vomiting, call a doctor or clinic for a prescription.