How Long Does an Allergic Reaction Last?

Medically Reviewed on 2/1/2023
Allergic Reaction
Severe allergic reactions (anaphylaxis) can be life-threatening and last for several days.

An allergic reaction can last anywhere from a few minutes to several days, depending on the severity and type of reaction. It may take anywhere from a few hours to a few days to naturally disappear. Some allergic reactions may take two to four weeks to resolve, even with appropriate treatment.

  • Mild allergic reactions, such as hives, hay fever, or nasal congestion, may resolve within a few hours to a day either with treatment such as medication or allergy shots or subside naturally.
  • Severe allergic reactions, such as anaphylaxis, are potentially fatal and necessitate immediate medical attention, as well as hospitalization for observation and treatment. Symptoms of severe allergic reactions may take several days or longer to resolve, even with prompt medical treatment.

The length of time it takes for an allergic reaction to go away can vary depending on several factors, including the type and severity of the reaction, the individual's immune system, and the type of treatment received. Some allergic reactions may reappear continuously during particular seasons, such as spring pollen season, and may give the impression that the allergic reactions last longer than usual.

If you suspect that you or someone you know is experiencing an allergic reaction, seek medical attention immediately to receive appropriate treatment.

For individuals with chronic allergies, treatment may involve taking preventive measures, such as avoiding triggers and taking medications on a regular basis to manage symptoms.

Follow your doctor's recommendations and continue to monitor your symptoms to ensure proper healing and minimize the risk of future reactions.

What is an allergic reaction?

Allergic reactions may last for varying lengths of time. They may take a few hours to a few days to disappear. If the exposure to the allergen continues, such as during a spring pollen season, allergic reactions may last for longer periods such as a few weeks to months.
Allergic reactions may last for varying lengths of time. They may take a few hours to a few days to disappear. If the exposure to the allergen continues, such as during a spring pollen season, allergic reactions may last for longer periods such as a few weeks to months.

An allergic reaction occurs when the body's immune system mistakenly identifies a harmless substance (allergen) as a threat and triggers various reactions as a response against it. These immune responses cause the release of chemicals, such as histamine, that trigger symptoms such as itching, redness, rashes, runny nose, swelling, and, in severe cases, anaphylaxis (a life-threatening reaction), leading to serious symptoms such as difficulty breathing.

The severity of an allergic reaction can range from mild to life-threatening.

Mild reactions, such as hay fever or hives, can cause discomfort but are usually not dangerous.

Severe reactions, such as anaphylaxis, can be life-threatening and require immediate medical attention.

Common allergens include the following:

  • Pollen
  • Dust mites
  • Pet dander
  • Certain foods (such as peanuts, tree nuts, shellfish, and eggs)
  • Insect stings
  • Certain medications

Allergic reactions are quite common and may occur seconds to hours after contact with the allergen. Although many allergic reactions are mild, others may be dangerous or life-threatening. They may be localized, involve a small part of the body, or affect a large area or the whole body.

Individuals with a known history of severe allergic reactions are highly recommended to carry emergency medication, such as an epinephrine auto-injector, and must inform others of their allergies. Others living with such individuals must know how to respond in the event of a reaction.

Are allergies and hypersensitivity reactions the same?

Usually, the words allergic reactions and hypersensitivity are used synonymously to describe an overreaction of the immune system to a substance that is normally harmless. However, there is a slight difference between them.

Allergic reaction is a broad term used to describe any immune response to an allergen that could be mild to severe, whereas hypersensitivity reactions specifically refer to an abnormal immune response to an allergen and are of excessive intensity.

All hypersensitivity reactions are allergic reactions, but not all allergic reactions are hypersensitivity reactions. For example, you may have a mild allergic reaction to pollen (such as hay fever), which would still be considered an allergic reaction, but not necessarily a hypersensitivity reaction, as the intensity of the reaction is not excessive.


Common Allergies: Symptoms and Signs See Slideshow

What are the signs and symptoms of an allergic reaction?

A girl with allergic reactions
The typical symptoms of allergic reactions are Itching, skin rash, redness or hives, sneezing, running/blocked nose, and more.

Symptoms of allergic reactions vary depending on whether the allergen came in contact with the skin, was inhaled, swallowed, or injected, as well as the type and severity of the reaction.

Some common symptoms of allergic reactions include:

Seek emergency care if you experience any of the following symptoms during an allergic reaction as they indicate a severe life-threatening allergic reaction:

How to differentiate between a severe and a common allergic reaction

Symptoms of severe allergic reactions (anaphylaxis) can be life-threatening compared with those of common allergic reactions.

Some of the differences between severe allergic reactions and common allergic reactions include:

  • Speed of onset
    • Anaphylaxis can develop rapidly, within minutes to an hour after exposure to the allergen.
    • Common allergic reactions may take several hours to develop.
  • Symptoms
    • Common allergic reactions may cause symptoms such as:
      • Itching
      • Hives
      • Runny nose
      • Sneezing
    • Anaphylaxis can cause serious symptoms such as:
      • Difficulty breathing
      • Rapid heartbeat
      • Dizziness
      • Fainting
      • Swelling of the face, lips, and tongue
  • Whole-body involvement
    • Common allergic reactions may be localized to one part of the body, such as the skin or respiratory system.
    • Anaphylaxis can involve multiple body systems, such as the respiratory, cardiovascular, and digestive systems.
  • Need for medical intervention
    • Common allergic reactions may be treated with over-the-counter medications, such as antihistamines, and do not usually require medical intervention.
    • Anaphylaxis is a medical emergency and requires prompt treatment with epinephrine (an adrenaline shot) and immediate transport to the hospital.

What are the most common allergens causing allergic reactions?

Some individuals may develop allergic reactions to various substances that occur in nature. The severity of the reaction depends on various factors including their genetic makeup, geographical location, and others.

Some common allergens causing allergic reactions include:

  • Pollen from various
    • Plants
    • Trees
    • Grass
    • Weeds
  • Dust mites
  • Pet dander (tiny flakes of skin or hair) from dogs, cats, and other animals
  • Mold spores
  • Certain foods
    • Peanuts
    • Tree nuts
    • Milk
    • Eggs
    • Wheat
    • Soy
    • Seafood
  • Insect stings, such as bee stings
  • Certain medications, such as penicillin
  • Latex
  • Cosmetics
  • Laundry detergents

3 stages of an allergic reaction

It is not necessary for every individual to experience all the stages. The severity and progression of symptoms can vary greatly from person to person. Some individuals may experience severe, life-threatening anaphylactic reactions immediately after exposure to the allergen and require immediate medical attention.

  1. Prodrome: The first stage of an allergic reaction and usually occurs within minutes of exposure to the allergen. During this stage, the body releases histamine and other chemicals, causing mild symptoms such as itching, redness, and swelling.
  2. Early reaction: Occurs within minutes to hours after exposure to the allergen. During this stage, symptoms may become more severe, including hives, nasal congestion, wheezing, and abdominal pain.
  3. Late reaction: Occurs several hours to days after exposure to the allergen. During this stage, symptoms such as fatigue, headache, and muscle aches may develop. In some individuals, this stage may also be accompanied by a recurrence of skin symptoms, such as hives or itching.

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4 types of allergic reactions

  1. Immediate type hypersensitivity (type I): Most common type of allergic reaction and occurs within minutes to hours after exposure to an allergen. This type of reaction is characterized by symptoms such as itching, hives, redness, swelling, nasal congestion, wheezing, and abdominal pain.
  2. Cytotoxic (type II): This occurs when the immune system attacks cells, leading to the destruction of these cells. This type of reaction is often seen in response to blood transfusions or organ transplants, where the body recognizes the foreign cells as a threat and mounts an immune response.
  3. Immune complex (type III): Occurs when the immune system produces antibodies against an allergen, forming immune complexes that can cause inflammation and damage to tissues. This type of reaction is often seen in response to allergens such as insect bites and certain medications and can cause symptoms such as joint pain, skin rashes, and eye inflammation.
  4. Delayed type hypersensitivity (type IV): Occurs several hours to days after exposure to an allergen and is characterized by symptoms such as fatigue, headache, muscle aches, and skin rashes. This reaction is often seen in response to allergens such as poison ivy, metal implants, and certain medications.

You may experience more than one type of allergic reaction at a given time when exposed to an allergen involved. Seek medical attention immediately after you experience an allergic reaction.

How is an allergic reaction diagnosed?

To determine a possible allergic reaction caused by an allergen, your doctor will typically note down all the symptoms, complete your medical history and perform a physical examination looking for signs of an allergic reaction, such as hives or red, itchy skin.

To confirm the diagnosis, you will be subjected to various tests, which include:

  • Skin prick test: A small amount of allergen is placed on the skin, and a needle is used to prick the skin. A reaction at the site of the prick can indicate an allergy.
  • Blood test: A blood sample is taken and tested for levels of immunoglobulin E antibodies, which are produced in response to allergens.
  • Elimination diet: The person is asked to remove suspected allergens from their diet for a certain period and monitor symptoms to see if they improve.
  • Food challenge test: The person is given small amounts of a suspected allergen to see if it triggers a reaction.

The results generated by these tests can help determine the presence of allergic reactions and even specify the allergen causing them. These test results can help determine the appropriate treatment options.

What are the treatment options for allergic reactions?

Various treatment options are available for allergic reactions, and the most suitable treatment is chosen based on the type and severity of the symptoms.

  • Medications
    • Over-the-counter or prescription medications, such as antihistamines, decongestants, or corticosteroids, can help relieve symptoms of an allergic reaction.
    • Diphenhydramine is an over-the-counter antihistamine that can help relieve symptoms of mild to moderate allergies, such as itching, sneezing, runny nose, and hives. It works by blocking the effects of histamine—a chemical released by the body during an allergic reaction. It may cause drowsiness and should be used with caution if you are operating heavy machinery or driving and may interact with other medications.
  • Epinephrine
    • Can quickly reverse symptoms of severe and potentially life-threatening reactions such as anaphylaxis by narrowing blood vessels, relaxing muscle tissues, and increasing heart rate.
  • Bronchodilators
    • Can help relax the muscles around the airways and improve breathing in asthmatics.
  • Cool compress
    • Applying a cool compress to the affected area can help soothe skin irritation and reduce itching.
  • Bath or shower
    • Taking a bath or shower with cool water can help wash away allergens and soothe skin irritation.
  • Hydration
    • Staying hydrated by drinking plenty of water can help flush allergens out of your system and prevent dehydration.
  • Alternative treatments
    • Some individuals may choose to use alternative treatments, such as herbal remedies or acupuncture, to manage symptoms of an allergic reaction. However, these treatments are not always proven to be beneficial and should only be done under the supervision of a professional.

What is the immediate action to be taken for an allergic reaction?

You must follow certain guidelines immediately if you or someone you know experiences anaphylaxis to prevent potential life-threatening consequences.

  • Call for emergency services: Anaphylaxis is a medical emergency that requires immediate treatment.
  • Administer epinephrine: If you have been prescribed an epinephrine auto-injector, use it as directed by your doctor. This medication can help quickly reverse symptoms of anaphylaxis.
  • Lie down and elevate your feet: Lie down with your feet elevated to help improve blood flow and slow down the progression of symptoms.
  • Stay calm: Try to remain calm and avoid activities that can increase your heart rate and worsen symptoms, such as running or jumping.
  • Wait for medical help: After administering epinephrine and lying down, wait for medical help to arrive. Continue to monitor your symptoms and take additional doses of epinephrine if symptoms worsen or return.

What precautions can help prevent possible allergic reactions?

Preventing allergies involves reducing exposure to allergens and taking steps to manage symptoms when they occur. If you have a history of allergic reactions, you may follow several precautions to help minimize your symptoms and reduce your risk of an allergic reaction.

The following are a few guidelines that can help prevent allergies:

  • Identifying and avoiding allergens
    • Identify the specific allergens that trigger your symptoms and avoid exposure to them. This can help prevent allergic reactions from occurring.
    • If you have food allergies, read food labels carefully to avoid ingredients that cause allergic reactions.
  • Keep your surroundings clean
    • Dust and vacuum your home regularly to reduce allergens in the air.
    • Use air purifiers and keep windows and doors closed to reduce outdoor allergens.
    • Wash bedding, curtains, and stuffed toys frequently to reduce dust mite allergens.
  • Take medications as directed
    • Taking over-the-counter or prescription medications, such as antihistamines, nasal sprays, or decongestants, as directed by your doctor can help manage symptoms and prevent reactions.
    • Allergy shots or immunotherapy can also reduce symptoms as it builds up immunity to specific allergens over time, reducing the possibility of an allergic reaction.
  • Wash hands frequently
    • Frequent handwashing, especially after being outside or coming into contact with potential allergens, can help reduce exposure to allergens and prevent allergic reactions.
  • Wear protective clothing
    • If you have outdoor allergies, wear a hat, sunglasses, and a mask to reduce exposure to airborne allergens, especially in a public place or crowded area.
    • Wear a long-sleeved shirt, pants, and gloves when mowing the lawn or doing other outdoor activities to reduce skin exposure to allergens.
  • Maintaining a healthy diet
    • Eating a balanced and nutritious diet rich in fruits and vegetables can help strengthen your immune system and reduce the likelihood of developing allergies.

Can you become allergic to something suddenly?

It is possible for anyone to develop an allergy suddenly. This type of allergy is known as acute onset allergy and can occur within minutes or hours of exposure to an allergen.

Common allergens that can cause sudden-onset allergies to include the following:

  • Certain foods
  • Insect stings
  • Medications

An individual may have been exposed to the same allergen multiple times before developing an allergy, whereas some may have an allergic reaction immediately after exposure to an allergen for the very first time.

The development of an allergy is influenced by a combination of genetic and environmental factors. If you suspect that you have developed a sudden onset allergy, seek medical attention for proper diagnosis and treatment.

Medically Reviewed on 2/1/2023
Drug Allergies:

Serious Allergic Reactions (Anaphylaxis):

Anaphylaxis: First aid: anaphylaxis/basics/art-20056608

Drug Allergy: information/conditions-treated-a-to-z/drug-allergy