A delayed allergic reaction might occur an hour or more after you’ve been exposed to an allergen. You might also experience a biphasic reaction, in which a second reaction occurs long after the first.
If you have a peanut allergy, your immune system will start an attack anytime it senses the proteins in peanuts. This will cause the release of chemicals that trigger symptoms such as itchy hives, nausea, and facial swelling. Peanut allergies are common in the United States.
Some people have severe peanut allergies. When they’re exposed to even the tiniest trace of peanuts, they develop a life threatening total-body reaction called anaphylaxis.
An anaphylactic reaction often starts within seconds after someone with a severe allergy eats peanuts. However, in rare cases, symptoms can be delayed, appearing minutes or hours after exposure.
Even if you’ve been treated for a severe reaction and now feel fine, you could develop a second reaction hours or days later without being exposed to peanuts again. This is called biphasic anaphylaxis.
Read on to find out why delayed and biphasic reactions are so dangerous and how to prevent them from happening to you or your child.
The symptoms of a delayed anaphylactic reaction can show up
Common anaphylaxis symptoms include:
- swelling of your face, eyes, lips, or throat
- wheezing or trouble breathing
- a weak, fast pulse
- pale skin
- confusion
- a sudden feeling of warmth
- dizziness or fainting
- itchy skin
- hives
- vomiting
- diarrhea
- cramps
The symptoms of a delayed reaction may be more severe or less severe than those of an immediate reaction.
Delayed reactions are generally rare. They occur mostly in children with food allergies and in people who have a type of red meat allergy that can develop as a result of a bite from a lone star tick.
You’re more likely to experience a delayed reaction if you:
- have a severe peanut allergy
- don’t get treated with epinephrine quickly enough
- don’t receive a large enough dose of epinephrine
- don’t respond quickly to epinephrine
- have low blood pressure during your first reaction
- have a history of delayed anaphylaxis
Biphasic reactions, in which a second reaction occurs after the initial symptoms have resolved, may occur in
A 2020 analysis of data from 9,171 people who had experienced anaphylaxis found that 4.7% of those reactions were biphasic. In most cases, the second reaction began within 12 hours of the first.
In a 2024 study involving 280 children who experienced anaphylaxis, about 1 in 7 had a second reaction after they had received an epinephrine injection to treat the initial reaction.
However, note that these studies included reactions to several types of allergens, not just peanuts.
Some allergic reactions are mild, but anaphylaxis is a very serious condition. Your airways can tighten so much that you can’t breathe. Someone who is experiencing anaphylaxis can die within 15 to 30 minutes if they don’t get medical help.
And in some cases, people who have been treated for an allergic reaction and seem fine can develop a second reaction hours later.
In 2013, 13-year-old Natalie Giorgi ate a small bite of a dessert that contained peanuts while on summer vacation with her family. She received three doses of epinephrine and seemed fine afterward, but she died of a severe allergic reaction later that evening.
If you know you have a severe peanut allergy, the best way to prevent anaphylaxis is to avoid exposure to peanuts. Here are a few tips:
- Read food labels carefully when shopping: Any packaged food that contains peanuts must mention this on the list of ingredients.
- When ordering food, always tell the server that you have a peanut allergy: Ask for your food to be prepared without peanuts, peanut oil, or other peanut-based products.
- When traveling by plane, tell the airline about your allergy ahead of time: You can request that your flight be free from peanuts and that your seat be cleaned.
As a precaution, always keep an epinephrine auto-injector (often known by the brand name EpiPen) nearby. Epinephrine can reverse the symptoms of an anaphylactic reaction, but you have to use it quickly for it to be effective.
During a delayed reaction, you may need a second and possibly a third dose of epinephrine. An allergist can show you how to use the auto-injector correctly.
After you inject epinephrine and your symptoms stabilize, go to an emergency room for treatment. Always get medical help to prevent another reaction.
If you’ve had an allergic reaction to peanuts, you need to consult an allergist. They’ll review your medical history and symptoms, give you tips on how to avoid peanuts, and determine whether you need to keep an epinephrine auto-injector on hand for emergencies.
A reaction to any allergen, including peanuts, can sometimes be delayed, occurring an hour or longer after exposure. It’s also possible to experience a second reaction after the first one has resolved.
Always go to the emergency room after an anaphylactic reaction, even if you’ve used epinephrine and your symptoms have subsided. Healthcare professionals may need to monitor you for some time in case you have a second reaction.