Penicillin allergy

Understand the importance of an accurate diagnosis for this type of allergy and the best practices for antibiotic treatment.

Update Date: 20.09.2025

Overview
Symptoms
When to see a doctor
Causes
Risk factors
Prevention
Diagnosis
Treatment
Preparing for your appointment

Overview

Penicillin allergy is a reaction of the immune system to the antibiotic medicine penicillin. Penicillin is used to treat bacterial infections.

Penicillin allergy may cause hives, rash and itching. A serious reaction can include anaphylaxis. Anaphylaxis is a life-threatening condition that can cause throat swelling and trouble breathing.

Research has shown that penicillin allergies may be over-reported. When allergies are over-reported, it can result in the use of improper and more-expensive antibiotic treatments. Therefore, to ensure the best treatment options in the future, an accurate diagnosis is needed when penicillin allergy is suspected.

Other antibiotics, such as those similar to penicillin, also can result in allergic reactions.

Symptoms

Symptoms of penicillin allergy often happen within an hour after taking the antibiotic. Less commonly, reactions can happen hours, days or weeks later.

Penicillin allergy symptoms may include:

  • Skin rash.
  • Hives.
  • Itching.
  • Fever.
  • Swelling.
  • Shortness of breath.
  • Wheezing.
  • Runny nose.
  • Itchy, watery eyes.
  • Anaphylaxis.

Anaphylaxis

Anaphylaxis is a rare, life-threatening allergic reaction that causes problems in multiple parts of the body. Symptoms of anaphylaxis include:

  • Tightening of the airways and throat, causing breathing problems.
  • Nausea or stomach cramps.
  • Vomiting or diarrhea.
  • Dizziness or lightheadedness.
  • Weak, rapid pulse.
  • Drop in blood pressure.
  • Seizures.
  • Loss of consciousness.

Delayed reactions resulting from penicillin allergy

Less common penicillin allergy reactions happen days or weeks after exposure. Reactions may continue for some time after you stop taking the antibiotic. These conditions include:

  • Serum sickness, which may cause fever, joint pain, rash, swelling and nausea.
  • Anemia. Anemia is a reduction in red blood cells caused by the medicine. It can cause fatigue, irregular heartbeats, shortness of breath and other symptoms.
  • Drug reaction with eosinophilia and systemic symptoms, also called DRESS. DRESS causes rash, high white blood cell counts, general swelling and swollen lymph nodes. DRESS also can cause hepatitis infections to come back.
  • Stevens-Johnson syndrome or toxic epidermal necrolysis, which involves serious blistering and peeling of the skin.
  • Inflammation in the kidneys, called nephritis. Nephritis can cause fever, blood in the urine, general swelling, confusion and other symptoms.

Adverse events that are not allergic reactions

You may experience side effects of penicillin that are not an allergic reaction to the antibiotic. This also can happen with other medicines. Depending on the type of penicillin, common side effects may include mild nausea or diarrhea, headache, or vaginal itching. Unrelated symptoms or symptoms of an infection for which you are being treated also may be mistaken as an allergic reaction to medicine.

When to see a doctor

Make an appointment with your doctor or other healthcare professional if you have any lasting symptoms that worry you. It's important to understand and discuss what is an allergic reaction, what is a typical side effect and what you can tolerate in taking a medicine.

Call 911 or emergency medical help if you experience symptoms of a serious reaction or possible anaphylaxis after taking penicillin.

Drug rash

A medicine rash usually starts within two weeks of taking a new medicine. The rash begins as small spots that spread. It can cover large areas of the body.

Causes

Penicillin allergy happens when the immune system becomes overly sensitive to the antibiotic. The immune system reacts to the medicine as if it were a harmful substance, such as a viral or bacterial infection.

Before the immune system can become sensitive to penicillin, you have to be exposed to the medicine at least once. If and when your immune system thinks penicillin is a harmful substance, it develops an antibody to the medicine.

The next time you take the medicine, these specific antibodies flag it and attack the substance. Chemicals released by this activity cause the symptoms related to an allergic reaction.

Being exposed to penicillin in the past may not have been something you noticed. Some evidence suggests that very small amounts of penicillin in the food supply may be enough for the immune system to create an antibody to it.

Penicillins and related medicines

Penicillins belong to a class of antibacterial medicines called beta-lactam antibiotics. Although these medicines are slightly different from one another, they generally fight infections by attacking the walls of bacterial cells. In addition to penicillins, other beta-lactams more commonly related to allergic reactions are a group called cephalosporins.

If you've had an allergic reaction to one type of penicillin, you could be allergic to other types of penicillin or to some cephalosporins.

Penicillins include:

  • Amoxicillin.
  • Ampicillin.
  • Dicloxacillin.
  • Nafcillin.
  • Oxacillin.
  • Penicillin G.
  • Penicillin V.
  • Piperacillin.
  • Ticarcillin.

Cephalosporins include:

  • Cefaclor.
  • Cefadroxil.
  • Cefazolin.
  • Cefdinir.
  • Cefepime.
  • Cefotetan.
  • Cefprozil.
  • Cefuroxime.
  • Cephalexin.

Risk factors

While anyone can have an allergic reaction to penicillin, a few factors can increase the risk. These include:

  • A history of other allergies, such as food allergy or hay fever.
  • An allergic reaction to another medicine.
  • A family history of medicine allergy.
  • Increased exposure to penicillin because of using high doses or using the medicine often or over long periods of time.
  • Certain illnesses commonly related to allergic medicine reactions, such as infection with HIV or the Epstein-Barr virus.

Prevention

If you have a penicillin allergy, the simplest prevention is to avoid the medicine. Steps you can take to protect yourself include the following:

  • Inform healthcare workers. Be sure that your penicillin allergy or other antibiotic allergy is clearly identified in your medical records. Inform other healthcare professionals, such as your dentist.
  • Wear a bracelet. Wear a medical alert bracelet that identifies your allergy. This information can ensure proper treatment in an emergency.

Diagnosis

A thorough exam and proper diagnostic tests are needed for an accurate diagnosis. A misdiagnosed penicillin allergy may result in the use of improper or more expensive antibiotics.

Your healthcare professional conducts a physical exam and ask questions about your symptoms. Your health professional also may order tests. You may be referred to an allergy specialist, called an allergist, for these tests. Tests may include the following.

Skin tests

With a skin test, the allergist or nurse puts a small amount of the suspect penicillin in your skin with a tiny needle. A positive reaction to a test causes a discolored, itchy, raised bump.

A positive result means the likelihood of penicillin allergy is high. A negative test result usually means you're not at high risk of an allergy to penicillin. But a negative result is more difficult to understand because some kinds of medicine reactions cannot be detected with skin tests.

Graded challenge

If the diagnosis of a penicillin allergy is uncertain, a graded challenge may be recommended. With this procedure, you receive up to five doses of the suspect penicillin. The challenge starts with a small dose and increases to the desired dose. If you reach the desired dose with no reaction, your healthcare professional concludes that you aren't allergic to that type of penicillin. You will be able to take the antibiotic as prescribed.

If you are allergic to one type of penicillin, you may have to do a graded challenge with a type of penicillin or cephalosporin that's less likely to cause an allergic reaction. This would help your healthcare professional find an antibiotic that you can safely use.

During a graded challenge, you're carefully monitored. Supportive care services also are available to treat a reaction.

Treatment

Treatments for a penicillin allergy can be divided into two general strategies:

  • Treatment for the current allergy symptoms.
  • Treatment to lessen sensitivity to penicillin, called desensitization.

Treating current symptoms

The following treatments may be used for symptoms of an allergic reaction to penicillin:

  • Withdrawal of the medicine. If your healthcare professional determines that you have or likely have a penicillin allergy, stopping the medicine is the first step in treatment.
  • Antihistamines. You may be prescribed an antihistamine. You also may use an antihistamine that you can buy without a prescription. Antihistamines can block the immune system during an allergic reaction.
  • Corticosteroids. Either oral or injected corticosteroids may be used to treat inflammation related to more-serious reactions.
  • Treatment of anaphylaxis. Anaphylaxis requires an immediate treatment with medicine as well as hospital care to maintain blood pressure and support breathing.

Medicine desensitization

If there are no other suitable antibiotic treatment options available, your healthcare professional may recommend a treatment called drug desensitization. This allows you to take a course of penicillin to treat an infection. With this treatment, you receive a very small dose of the medicine. The dose gets larger every 15 to 30 minutes over the course of several hours or a few days. If you can reach the desired dose with no reaction, you can continue the treatment.

It's important to take the medicine as directed to maintain your tolerance to it during the entire course of treatment. If you need penicillin in the future, you will likely need to repeat the desensitization treatment.

You're carefully monitored during the treatment. Supportive care also is available to treat reactions. Desensitization is not always successful, and there is a risk of serious reactions.

Preparing for your appointment

Be prepared to answer questions, such as:

  • What symptoms did you experience?
  • What is the name of the penicillin or other antibiotic you were taking?
  • Why were you prescribed the antibiotic?
  • Have you had these symptoms in the past when you weren't taking this medicine?
  • How long after taking penicillin did the symptoms begin?
  • How long did the symptoms last?
  • Have you stopped taking the medicine?
  • What other medicines, herbal medicines, vitamins or other dietary supplements do you take?
  • At what time of day do you take your other medicines or supplements?
  • Have you increased the dosage of any regular medicine or supplement?
  • Have you stopped taking your regular medicines or supplements?
  • Did you take anything to treat your symptoms? If so, what was the effect?
  • Have you had a reaction to any medicine in the past? If so, what medicine was it?
  • Do you have hay fever, food allergy or other allergies?
  • Is there a history of penicillin or other medicine allergies in your family?

You may want to take pictures of any condition, such as a rash or swelling, to show your healthcare professional. Pictures may help your healthcare professional if symptoms have gotten better by the time of your appointment.

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