The most common triggers of anaphylaxis are medications (like antibiotics or NSAIDs), stinging insects (like bees, wasps, or fire ants), and foods (like shellfish, peanuts, or tree nuts). But in up to 20% of cases, the trigger is unknown.
Anaphylaxis can happen quickly after exposure to the allergen, normally within one to two hours, and sometimes within minutes. It can also reoccur up to 3 days after exposure, which is why people at high risk should be monitored closely after a severe allergic event.