How Long Does an Allergic Reaction Last?

Quick Answer

Mild allergic reactions typically resolve in 1–2 hours with antihistamines. Moderate reactions involving hives or swelling can last 1–3 days. Severe anaphylaxis is a life-threatening emergency that develops within minutes and requires immediate epinephrine injection. The total duration depends on the allergen, the severity of your immune response, and how quickly you begin treatment.

Duration by Reaction Type

Reaction Type	Typical Duration	Common Symptoms
Mild	1–2 hours	Localized itching, minor hives, sneezing, runny nose
Moderate	1–3 days	Widespread hives, swelling, nasal congestion, mild wheezing
Severe (anaphylaxis)	Minutes to hours	Throat swelling, difficulty breathing, drop in blood pressure, loss of consciousness
Contact dermatitis	2–4 weeks	Itchy rash, blisters, dry cracked skin at contact site

Duration by Allergen

Food allergies produce symptoms within minutes to 2 hours of ingestion. Mild food reactions resolve in a few hours once the allergen passes through your system, but severe food reactions can trigger biphasic anaphylaxis, where symptoms return 4–12 hours after the initial reaction.

Drug allergies vary widely. A reaction to an antibiotic like penicillin may appear within an hour or take up to two weeks to develop. Once the medication is stopped, symptoms typically clear within 3–7 days, though some drug rashes last 2–3 weeks.

Insect sting allergies cause immediate pain and localized swelling lasting 24–72 hours. Large local reactions (swelling exceeding 10 cm) can persist for 5–10 days. Anaphylaxis from insect stings peaks within 5–30 minutes.

Contact allergies (poison ivy, nickel, latex) trigger delayed reactions appearing 12–72 hours after exposure. The resulting dermatitis lasts 2–4 weeks even with treatment, making contact allergies the longest-lasting type.

Antihistamine Timeline

Over-the-counter antihistamines like diphenhydramine (Benadryl) begin working within 15–30 minutes and provide relief for 4–6 hours. Second-generation antihistamines like cetirizine (Zyrtec) or loratadine (Claritin) take 1–3 hours to reach full effect but last 24 hours with fewer sedating side effects. For moderate reactions, doctors may prescribe a short course of oral corticosteroids, which reduce inflammation over 3–5 days.

When to Use an EpiPen

Use epinephrine (EpiPen) immediately if you experience any of these anaphylaxis signs:

• Swelling of the tongue or throat
• Difficulty breathing or wheezing
• Dizziness, fainting, or rapid pulse
• Nausea, vomiting, or abdominal cramps combined with skin symptoms
• A known severe allergen exposure

Epinephrine works within 1–5 minutes but wears off in 15–20 minutes. Always call 911 after using an EpiPen because symptoms can return. Up to 20% of anaphylaxis cases involve a biphasic reaction requiring a second dose.

When to See a Doctor

• Symptoms don't improve after 2 doses of antihistamine
• Hives or swelling last longer than 48 hours
• You experience any difficulty breathing
• You've had to use your EpiPen
• Allergic reactions keep recurring without a clear trigger

An allergist can perform skin prick or blood tests to identify specific triggers, which helps you avoid future reactions and shorten recovery times.