How Long Can Allergic Reactions Last?

Allergic reactions can last anywhere from a few minutes to several weeks, depending on the type of allergen involved, the nature of the reaction, and whether exposure to the trigger continues. Mild symptoms — such as sneezing, itching, or a few hives — often settle within hours once the allergen is removed or avoided. However, some reactions, particularly those affecting the skin (such as contact dermatitis or eczema flares), can persist for days or even weeks after the initial exposure.

This article provides general information about how long different types of allergic reactions typically last, what may cause some reactions to persist longer than expected, and when it is important to seek medical help. It is not a substitute for urgent medical care. If you are experiencing difficulty breathing, throat swelling, or collapse, call 999 immediately.

Quick Answer: How Long Do Allergic Reactions Usually Last?

Minutes to hours: Mild food or insect sting reactions often settle relatively quickly once exposure stops.
Hours to days: Hives (urticaria) often settle within a few days, though individual welts may appear and fade within hours.
Days to weeks: Contact dermatitis and some skin reactions can take one to three weeks to fully resolve.
Immediate emergency: Anaphylaxis can progress rapidly and requires immediate treatment — call 999.

Typical Allergic Reaction Timelines by Type

The table below provides a general overview of how long different types of allergic reactions may last. These are approximate ranges based on common clinical patterns — individual experiences can vary, and these timelines should not be used to self-diagnose or replace professional assessment.

Trigger / Type	Onset	Typical Duration	Key Notes
Food allergy (mild–moderate)	Often within minutes to 2 hours	Hours to 1–2 days	Skin and gut symptoms may settle at different rates
Insect sting allergy	Usually within minutes	Hours to several days (local swelling)	Large local reactions may persist for 5–10 days
Medication allergy (immediate)	Often within 1 hour	Hours to days	Seek urgent help if breathing or swelling involved
Medication allergy (delayed)	Days to weeks after starting	Days to weeks after stopping	Rashes can take time to clear; seek clinical assessment
Contact dermatitis	Hours to days after contact	1–3 weeks or longer	Can recur if re-exposure occurs
Hives (acute urticaria)	Often within minutes to hours	Individual welts: hours; episode: days	If lasting >6 weeks, classified as chronic
Seasonal allergies (hay fever)	During pollen exposure	Throughout the pollen season (weeks to months)	Symptoms can persist for the duration of the relevant season

Why Do Some Allergic Reactions Last Longer Than Others?

Understanding why some reactions resolve quickly while others linger can help set realistic expectations. Several factors can influence how long an allergic reaction lasts.

Continued Exposure

One of the most common reasons for prolonged allergic symptoms is ongoing exposure to the trigger. For example, someone with hay fever may experience symptoms throughout the pollen season because the allergen is continuously present in the environment. Similarly, someone living with a pet they are sensitised to may experience persistent nasal or skin symptoms because pet dander is difficult to avoid completely within the home. In these situations, the immune system is being repeatedly triggered, which can prevent symptoms from fully settling.

Skin Inflammation

Reactions that involve the skin — particularly contact dermatitis and eczema flares — often take longer to resolve than other types of allergic symptoms. This is because the inflammatory process in the skin can continue even after the original allergen has been removed. The skin needs time to repair, and factors such as scratching, dryness, or secondary irritation can delay healing further. It is not unusual for contact dermatitis to take two to three weeks to fully resolve.

Delayed and Biphasic Reactions

Not all allergic reactions follow an immediate pattern. Delayed allergic reactions can occur hours or even days after exposure. This is particularly relevant for contact dermatitis (where symptoms typically appear 24 to 72 hours after skin contact) and some medication reactions.

A biphasic allergic reaction is a less common but important pattern in which a second wave of symptoms occurs after the initial reaction has appeared to settle. This second phase can occur anywhere from one to 72 hours after the first reaction. Biphasic reactions are most relevant in the context of severe allergic reactions (anaphylaxis) and are one of the reasons why individuals who experience anaphylaxis are typically observed in hospital for a period after treatment. While biphasic reactions are not common, being aware of the possibility is important so that worsening symptoms are not dismissed as unrelated.

How Long Do Common Allergy Symptoms Last?

Hives (Urticaria)

Hives are raised, itchy welts on the skin that can appear as part of an allergic reaction. Individual hives tend to appear and fade within a few hours — often moving to different areas of the body — but the overall episode can last for several days. If hives persist for less than six weeks, they are classified as acute urticaria. If they continue beyond six weeks, they are classified as chronic urticaria.

It is worth noting that chronic urticaria is not always caused by allergy. In many cases of chronic hives, no specific external trigger is identified, and the condition may be related to other immune or inflammatory processes. If hives are persistent, a clinical assessment can help determine whether further investigation is appropriate.

Skin Rashes

Contact dermatitis — caused by direct skin contact with an allergen or irritant — can take considerably longer to resolve than other allergic symptoms. The rash may appear hours to days after contact and can persist for one to three weeks, even after the trigger has been removed. Avoiding the irritant and keeping the affected skin moisturised can support healing, but recovery takes time.

Eczema flares triggered or worsened by allergen exposure can also be prolonged. Because eczema is influenced by multiple factors — including skin barrier function, environmental conditions, and immune activity — the duration of a flare can be unpredictable.

Digestive Symptoms

Gut symptoms associated with food allergy — such as nausea, vomiting, abdominal pain, or diarrhoea — typically begin relatively quickly after consuming the trigger food, often within minutes to two hours. In most cases, these symptoms settle within a few hours to a day or two once the food has been digested and cleared from the system. However, if the reaction was significant, some residual discomfort may last a little longer.

Breathing or Throat Symptoms

Respiratory symptoms such as coughing, wheezing, or nasal congestion can occur during an allergic reaction. In the context of seasonal allergies, these symptoms may persist for the duration of the pollen season. For reactions to a specific food or substance, breathing symptoms may settle within hours once exposure has stopped.

However, any breathing or throat symptoms that are severe, worsening, or associated with swelling require immediate emergency attention.

Call 999 immediately for:

Difficulty breathing or wheezing
Swelling of the throat, tongue, or lips
Collapse, dizziness, or feeling faint
Rapid progression of symptoms

If the person has an adrenaline auto-injector, it should be used as directed while waiting for emergency services.

Can an Allergic Reaction Last for Days?

Yes, some allergic reactions can last for several days — and in certain cases, even longer. This is particularly true for skin-based reactions. Contact dermatitis, for example, can take one to three weeks to fully resolve. Hives may persist for several days, with individual welts appearing and fading over the course of the episode.

Ongoing or repeated exposure to the allergen can also extend the duration of symptoms. If you are sensitised to something you encounter regularly — such as dust mites in bedding, pet dander in the home, or an ingredient in a skincare product — symptoms may persist until the exposure is reduced or removed.

If your symptoms are lasting longer than you would expect, are worsening rather than improving, or are accompanied by new or concerning features, clinical assessment may be appropriate. Prolonged symptoms can sometimes be caused by factors other than allergy, and a professional evaluation can help clarify what is going on.

When Should You Seek Medical Advice?

Emergency (999)

Seek immediate emergency help by calling 999 if you or someone else experiences:

Difficulty breathing, wheezing, or feeling unable to get enough air
Swelling of the lips, tongue, or throat
Collapse, feeling faint, or losing consciousness
A rapidly worsening reaction despite initial treatment
Severe drowsiness or unresponsiveness (particularly in a child)

These may indicate anaphylaxis, which is a medical emergency. If the person has an adrenaline auto-injector, use it as directed and call 999. Do not delay.

Same-Day Medical Advice

It is advisable to seek same-day assessment from an urgent care service, walk-in centre, or pharmacist if you experience:

Swelling that is worsening, even if breathing is not currently affected
Breathing symptoms that are mild but not resolving
A suspected allergic reaction following a new medication
Widespread hives accompanied by feeling generally unwell

Routine Assessment

A routine clinical assessment may be helpful if you are experiencing:

Recurrent symptoms that follow a recognisable pattern
Symptoms where the trigger is unclear
Reactions lasting longer than expected
Ongoing symptoms that are affecting your daily life or sleep

Understanding what you are reacting to can help inform avoidance strategies and symptom management. This is where allergy testing may play a role.

Where Does Allergy Blood Testing Fit In?

If you are experiencing recurrent or unexplained allergic symptoms, allergy blood testing can help identify whether you are sensitised to specific allergens. These tests measure specific IgE antibodies in the blood — proteins produced by the immune system in response to particular triggers such as pollen, dust mites, animal dander, or certain foods.

It is important to understand what these tests can and cannot tell you. A positive result indicates sensitisation, meaning your immune system has recognised and responded to a particular allergen. However, sensitisation does not always equate to clinical allergy, and blood test results cannot predict with certainty how severe a future reaction may be. Results are most useful when interpreted alongside your symptom history and clinical context, ideally with input from a qualified healthcare professional.

Allergy blood tests do not replace emergency care. They are an investigative tool that can support a broader assessment of recurrent symptoms — not a substitute for urgent medical treatment during an active reaction.

A nurse-led allergy blood test appointment typically includes:

A symptom history discussion to understand the clinical picture
Venous blood sample collection by a trained nurse
Laboratory analysis of specific IgE levels
Secure delivery of results
Guidance on appropriate next steps

Our clinic provides nurse-led venous blood sample collection. Samples are sent to a laboratory for analysis, and results are delivered securely to you. We do not provide diagnosis, prescriptions, or emergency care. View our allergy blood tests.

Practical Steps That May Help Symptoms Settle

While allergic reactions often resolve on their own once the trigger is removed, the following practical measures may help support symptom relief. These are general suggestions and should not replace professional advice.

Remove or avoid the suspected trigger: Where possible, stopping contact with the allergen is the most effective way to allow symptoms to settle.
Keep skin moisturised: If a rash or skin reaction is present, using an unfragranced emollient can help soothe the skin and support the healing process.
Keep a symptom diary: Recording what you eat, your environment, the timing of symptoms, and their duration can help identify patterns and potential triggers over time.
Speak to a pharmacist or clinician: Before starting or changing any medication — including over-the-counter antihistamines — it is advisable to seek guidance from a pharmacist or appropriately qualified clinician.

Frequently Asked Questions

How long do hives last after an allergic reaction?

Individual hives typically appear and fade within a few hours, but new welts may continue to develop as others resolve. An overall episode of acute urticaria often lasts from a few days up to several weeks. If hives persist beyond six weeks, they are classified as chronic urticaria, which may require further assessment. The duration can depend on the trigger and whether exposure is ongoing.

Can allergic reactions last a week?

Yes, some allergic reactions can last a week or longer. Skin-based reactions are most likely to persist for extended periods. Contact dermatitis, for example, can take one to three weeks to fully resolve. If the allergen remains present — such as in a skincare product still being used, or an environmental trigger like pet dander — symptoms may continue until the exposure is addressed. Persistent symptoms warrant professional assessment.

What is a delayed allergic reaction?

A delayed allergic reaction is one that appears hours or days after exposure rather than immediately. Contact dermatitis is a common example, with symptoms often developing 24 to 72 hours after skin contact with the trigger substance. Some medication reactions can also be delayed, appearing days or even weeks after starting a new medicine. Delayed reactions involve different immune pathways compared with immediate IgE-mediated reactions.

Can symptoms return after they go away?

Yes. In some cases, a biphasic reaction can occur, where symptoms appear to resolve and then return — sometimes within hours, and in some cases up to 72 hours later. This is most relevant in the context of severe allergic reactions. Additionally, if you are re-exposed to the same allergen, symptoms can recur. This is why identifying triggers through appropriate assessment can be helpful for long-term management.

How long does contact dermatitis last?

Contact dermatitis typically takes one to three weeks to fully resolve, even after the trigger has been removed. The skin needs time to repair from the inflammatory process. Factors such as continued scratching, use of irritating products, or re-exposure to the allergen can prolong healing. Keeping the skin moisturised with an unfragranced emollient and avoiding known irritants can support recovery.

Does an allergy blood test show how severe a reaction will be?

No. Allergy blood tests measure the level of specific IgE antibodies, which indicates whether sensitisation is present. However, IgE levels do not reliably predict the severity of future reactions. Someone with a relatively low IgE level can sometimes experience a significant reaction, while someone with a higher level may have mild symptoms. Results should always be interpreted alongside clinical history by a qualified professional.

When should I get tested for allergies?

Allergy testing may be worth considering if you are experiencing recurrent symptoms that follow a pattern, if the trigger for your reactions is unclear, or if symptoms are affecting your quality of life. It can also be useful if you have experienced a reaction and want to confirm which allergen was responsible. Testing is most informative when combined with a detailed symptom history and professional interpretation.

Can stress prolong allergic symptoms?

While stress does not directly cause allergic reactions, there is evidence to suggest that psychological stress can influence immune function and may affect how the body responds to allergens. Some people report that their allergy symptoms — particularly skin conditions like eczema or hives — seem to worsen during periods of stress. Managing stress through appropriate strategies may support overall wellbeing, though it is unlikely to be a standalone solution for allergic symptoms.

Summary

How long an allergic reaction lasts depends on many factors, including the type of allergen, the nature of the reaction, and whether exposure continues. Most mild reactions settle within hours to a few days. Skin-based reactions — particularly contact dermatitis — may take one to three weeks to fully resolve. Seasonal allergies can persist for the duration of the relevant pollen season.

Emergency symptoms such as difficulty breathing, throat swelling, or collapse require immediate help — always call 999 in these situations. For symptoms that recur or persist beyond expected timeframes, clinical assessment can help identify what is driving the reaction and what steps may be appropriate.

If you are experiencing recurring or unexplained symptoms, a nurse-led allergy blood test may help identify potential triggers. Results can support discussions with a qualified healthcare professional about your symptoms and next steps.

References

NHS — "Allergies" (NHS Health A to Z)
NHS — "Anaphylaxis" (NHS patient information)
NHS — "Hives (urticaria)" (NHS Health A to Z)
NHS — "Contact dermatitis" (NHS Health A to Z)
NICE Clinical Knowledge Summaries — "Urticaria"
British Society for Allergy and Clinical Immunology (BSACI) — guidelines on allergy diagnosis and management
Allergy UK — "Types of allergic reactions" (patient information)