What causes a person to throw up undigested food 12 hours after you eat?

Lots. If acutely, can be due to infection, e.G viral gastroenteritis, acute inflammation, say from alcohol. . A bigger issue if chronic or repeated. Can also be due to inflammation of stomach/duodenum from alcohol, meds e.G aspirin, nsaids, ulcers, tumors (not likely at age 18), diabetes, intestinal blockage, other. Recurrent? Other symptoms, like weight loss, definitely needs to be checked by md asap.