Started feeling nausea like i was gonna a vomit out of nowhere. took 8mg zofran 20min ago how long till it works? havent vomited yet but nauseous.help

Depends on why: Nausea and vomiting in a 20yo female can result from something as simple as an emotional reaction to very serious but rare emergency conditions. If it comes with diarrhea its usually food poisoning or a virus. Without diarrhea could be pregnancy, a UTI*, or other pelvic problem. If there is abdominal or pelvic pain or fever, go to the ER. If your body has good reason to vomit, Zofran wont stop it.