ER triggers for stomach flu: dehydration signs (scant or dark urine, dizziness on standing, confusion), blood in vomit or stool, severe localized pain, fever above 104 F, or 24 hours unable to keep sips down.
Everything short of that usually rides out at home in a few days. The home protocol: small frequent sips of oral rehydration solution, diluted sports drinks, or broth, then bland foods as appetite returns, with dairy and rich food last to come back. Skip anti-diarrheal medicines when there is blood or high fever. Lower thresholds apply to infants, the elderly, pregnant women, and the immunocompromised, who dehydrate faster and tolerate it worse. The pattern that is not stomach flu: pain that localizes (especially right-lower abdomen, the appendix zone), black tarry stool, or illness stretching past several days, all of which deserve medical eyes. After it passes, a couple of tired days are normal.
What to do
Common questions
When should I go to the ER for stomach flu?
Most stomach flu (viral gastroenteritis) is miserable but safe at home, and the ER threshold is about dehydration and a short list of danger signs, not about how bad you feel. The dehydration red flags, per MedlinePlus: very little or no urine, or urine that is dark and concentrated; dizziness or fainting when standing; a dry or sticky mouth; lethargy or confusion; in kids, no tears when crying, no wet diapers for 6-plus hours, or a sunken soft spot in infants. Those mean IV fluids are the treatment, which is an ER or urgent-care visit. The danger signs that suggest something other than simple gastroenteritis: blood in vomit or stool (or black, tarry stool), severe or localized abdominal pain (especially right-lower, which is appendix territory), fever above 104 F, vomiting that prevents keeping even small sips down for 24 hours, or symptoms lasting beyond a few days. High-risk people get lower thresholds: infants, elderly adults, pregnant women, and the immunocompromised dehydrate faster and tolerate it worse. What actually helps at home while you ride it out: small, frequent sips of oral rehydration solution (or diluted sports drinks, broth, ice pops), then bland food as appetite returns; skip anti-diarrheals if there is blood or high fever. Afterward, expect a couple of days of fatigue, and ease back into dairy and rich food last.