Anywhere from 30 minutes to weeks - the germ sets the clock, and the CDC publishes the onset table.
Staph toxins hit within 30 minutes to 8 hours, norovirus at 12 to 48 hours, salmonella at 6 hours to 6 days, and listeria can take one to four weeks. That range is why the last thing you ate is so often the wrong suspect. Symptoms last hours to days; bloody diarrhea, diarrhea over 3 days, fever above 102 F, uncontrolled vomiting, or dehydration signs are the see-a-doctor list.
What to do
Common questions
How long does food poisoning take to start?
Anywhere from 30 minutes to weeks, because food poisoning is not one illness - the germ sets the clock, and the CDC publishes a table of exactly these onset windows. The fast end is preformed toxins: staph food poisoning can hit 30 minutes to 8 hours after eating, with violent vomiting that ends within a day or two. The classic middle is norovirus, at 12 to 48 hours. Salmonella typically runs 6 hours to 6 days, and campylobacter 2 to 5 days. The slow, dangerous outlier is listeria, which can take one to four weeks - the reason pregnant people get specific food-avoidance lists. This variability explains why the "it must have been the last thing I ate" instinct so often blames the wrong meal: yesterday's lunch is a more likely suspect than tonight's dinner for most germs. Symptoms themselves last a few hours to several days, and the CDC's see-a-doctor list covers bloody diarrhea, diarrhea over 3 days, fever above 102 F, vomiting that blocks fluids, and dehydration signs. Pymander, a free AI doctor available 24/7 by text, can help you match your timeline to the likely germ.