What's Triggering Your Hives?

What Is Penicillin Allergy?

Since the 1940s, penicillin has been a go-to drug to clear up infections caused by bacteria. But some people get a bad reaction from taking it.

Your immune system is supposed to fight off the bacteria that make you sick. But sometimes your body fights the medicine itself. That’s what happens if you are allergic to penicillin. Your immune system thinks it’s an invader and wants to get rid of it.

Anyone could be allergic to this type of antibiotic, but you might be more likely to if you have:

If you’ve had to take penicillin often, for a long time or in high doses, you may also be more likely to have a bad reaction.

Doctors try to match the right antibiotic to the right sickness. That job is tougher if you have a penicillin allergy. You may want to get tested if you notice problems.

What Are the Symptoms?

You could notice some of these signs of an allergy within an hour of taking penicillin:

  • Coughing, wheezing, and shortness of breath
  • Fever
  • Hives (red bumps on your skin that might be itchy)
  • Itchy, watery eyes
  • Itchiness on other parts of your body
  • Runny nose
  • Swelling of your skin, often around your face
  • Tightness in your throat

In rare cases, you might have a severe reaction called anaphylaxis. You or someone near you should call 911 if:

  • Your belly hurts.
  • It’s hard to breathe.
  • You have diarrhea.
  • You’re dizzy or light-headed, or you pass out.
  • You have seizures.
  • Your throat or tongue swells up.
  • There’s tightness in your chest.
  • You throw up, or feel like you might.

Though it’s not common, some allergic reactions can happen days or weeks later. Here are the some of the symptoms you would want to look out for:

  • Your joints hurt.
  • You have swelling.
  • You have a rash.
  • You feel like you’re about to throw up.
  • You get very tired.
  • You have a fever.
  • You feel confused.
  • Your heartbeat seems “off.”
  • There’s blood in your pee.

What Happens at an Allergy Test?

Make an appointment to see your doctor, who will examine you and answer questions about what symptoms you’ve had and how long they lasted.

You may also get a skin test or a challenge test.

Skin Test

This is the most common kind of test. It takes about an hour.

First, your doctor will use a tiny needle to prick your forearm and give you a weak dose of penicillin. The needle will barely break your skin. If you have an allergy, you’ll get an itchy red bump, which looks like a mosquito bite, in about 15 minutes.

If you don’t get a bump, they will give you a dose of penicillin under the skin of your forearm. Again, if you get a bump within 15 minutes, you are allergic to penicillin.

If you still don’t get a bump, it’s likely you’re not allergic.

Just to be sure, your doctor might give you a regular dose of penicillin by mouth. You’ll stay in the office for about an hour. If you don’t have any symptoms to this dose, your doctor will let you know if you are in the clear.

Challenge Test

Most of the time, you may get a challenge test when you really need penicillin and your doctor doesn’t have a skin test.

For the challenge test, they will start you with a small dose. If you don’t have a reaction after 30 to 60 minutes, you’ll take a higher dose. You will work your way higher every 30 to 60 minutes until you take a full dose. It usually takes four to five doses.

If you don’t have symptoms after the full dose, you don’t have the allergy.

What’s the Treatment?

If you have taken penicillin without realizing you have an allergy, stop taking it.

Then, your doctor may prescribe a medicine called an antihistamine, such as diphenhydramine (Benadryl, Genahist, Naramin, Sominex, Unisom), to help with your symptoms. For more serious problems such as swelling, they might give you a medicine called a corticosteroid.

With anaphylaxis, they may give you a drug called epinephrine right away. You’ll spend some time in the hospital until your blood pressure and breathing are better.

Your Options If You Are Allergic

When you can’t take penicillin, you normally avoid it. Your doctor will try to find another kind of antibiotic.

If you really need penicillin, you may get a treatment called desensitization. You usually would get this only if you didn’t react with anaphylaxis previously.

In desensitization, your doctor will start you with a small dose of penicillin. If you don’t show allergy symptoms in 15 to 30 minutes, then you get a higher dose.

You get higher doses over a few hours or days. If you don’t have symptoms, then you can keep taking penicillin.

WebMD Medical Reference Reviewed by Kumar Shital, DO on July 18, 2020



UpToDate: “Patient information: Allergy to penicillin and related antibiotics (Beyond the Basics).”

American College of Allergy, Asthma & Immunology: “Drug Allergy.”

Mayo Clinic: “Penicillin Allergy.”

American Academy of Allergy, Asthma & Immunology: “Penicillin Allergy FAQ.”

CDC: “Management of Persons Who Have a History of Penicillin Allergy.”

U.S. Department of Veteran Affairs: “Penicillin Skin Testing: Frequently Asked Questions.”

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