Sudden vomiting in adults is most commonly caused by acute gastroenteritis (the "stomach bug"), which is often a viral infection like Norovirus or Rotavirus. It can also be triggered by food poisoning, where toxins from bacteria like Staphylococcus aureus or Salmonella cause a rapid reaction, sometimes within hours of eating. Other grounded causes include motion sickness, an adverse reaction to a new medication, or even severe migraine headaches. In more serious cases, sudden vomiting can be a "red flag" for conditions like appendicitis, a bowel obstruction, or a gallbladder attack, especially if accompanied by intense abdominal pain or a fever. Environmental factors like heatstroke or excessive alcohol consumption are also frequent culprits. A supportive peer observation: while most cases are "self-limiting" and resolve with rest and hydration within 24 hours, you should seek immediate medical attention if the vomiting is "projectile," contains blood, or is accompanied by a stiff neck and severe headache (which can indicate meningitis). Understanding the "trigger"—whether it was a specific meal or contact with a sick person—is the first step in identifying the cause.