Diagnosis

A thorough exam and appropriate diagnostic tests are essential for an accurate diagnosis. A misdiagnosed penicillin allergy may result in the use of less-appropriate or more-expensive antibiotics.

Your doctor will conduct a physical examination, ask questions about your symptoms and order additional tests. You may be referred to an allergy specialist (allergist) for these tests. These may include the following.

Skin tests

With a skin test, the allergist or nurse administers a small amount of the suspect penicillin to your skin with a tiny needle. A positive reaction to a test will cause a red, itchy, raised bump.

A positive result indicates a high likelihood of penicillin allergy. 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 interpret because some kinds of drug reactions cannot be detected by skin tests.

Graded challenge

If the diagnosis of a penicillin allergy is uncertain, a graded drug challenge may be recommended. With this procedure, you receive up to five doses of the suspect penicillin, starting with a small dose and increasing to the desired dose. If you reach the therapeutic dose with no reaction, then your doctor will conclude you aren't allergic to that type of penicillin. You will be able to take the drug as prescribed.

If you are allergic to one type of penicillin, your doctor may recommend a graded challenge with a type of penicillin or cephalosporin that's less likely — because of known chemical properties — to cause an allergic reaction. This would enable your doctor to identify an antibiotic that can be used safely.

During a drug challenge, your doctor provides careful supervision, and supportive care services are available to treat an adverse reaction.

More Information

Treatment

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

  • Treatment for the current allergy symptoms
  • Desensitization to penicillin

Treating current symptoms

The following interventions may be used to treat the symptoms of an allergic reaction to penicillin:

  • Withdrawal of the drug. If your doctor determines that you have a penicillin allergy — or likely allergy — discontinuing the drug is the first step in treatment.
  • Antihistamines. Your doctor may prescribe an antihistamine or recommend an over-the-counter antihistamine such as diphenhydramine (Benadryl) that can block immune system chemicals activated during an allergic reaction.
  • Corticosteroids. Either oral or injected corticosteroids may be used to treat inflammation associated with more-serious reactions.
  • Treatment of anaphylaxis. Anaphylaxis requires an immediate epinephrine injection as well as hospital care to maintain blood pressure and support breathing.

Drug desensitization

If there are no other suitable antibiotic treatment options available, your doctor may recommend a treatment called drug desensitization that may enable you to take a course of penicillin to treat an infection. With this treatment, you receive a very small dose and then progressively larger doses every 15 to 30 minutes over the course of several hours or a few days. If you can reach the desired dosage with no reaction, then you can continue the treatment.

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

You're carefully monitored during the intervention, and supportive care 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 the following questions. These details will be important in helping your doctor determine the cause of your symptoms.

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

You may want to take pictures of any condition, such as a rash or swelling, to show your doctor. These may help your doctor if symptoms have subsided by the time of your appointment.

Dec. 05, 2019
  1. Shenoy ES, et al. Evaluation and management of penicillin allergy. Journal of the American Medical Association. 2019; doi:10.1001/jama.2018.19283. Accessed Oct. 22, 2019.
  2. Drug allergies. American College of Allergy, Asthma & Immunology. https://acaai.org/allergies/types/drug-allergies. Accessed Oct. 22, 2019.
  3. Kufel WD, et al. Penicillin allergy assessment and skin testing in the outpatient setting. Pharmacy. 2019; doi:10.3390/pharmacy7030136. Accessed Oct. 22, 2019.
  4. Solensky R. Penicillin allergy: Immediate reactions. https://www.uptodate.com/contents/search. Accessed Oct. 22, 2019.
  5. Anaphylaxis. Merck Manual Professional Version. https://www.merckmanuals.com/professional/immunology-allergic-disorders/allergic,-autoimmune,-and-other-hypersensitivity-disorders/anaphylaxis?query=anaphylaxis. Accessed Oct. 23, 2019.
  6. Solensky R. Penicillin allergy: Delayed hypersensitivity reactions. https://www.uptodate.com/contents/search. Accessed Oct. 22, 2019.
  7. Pongdee T, et al. Evaluation and management of penicillin allergy. Mayo Clinic Proceedings. 2018; doi:10.1016/j.mayocp.2017.09.020. Accessed Oct. 24, 2019.
  8. Penicillins. Merck Manual Professional Version. https://www.merckmanuals.com/professional/infectious-diseases/bacteria-and-antibacterial-drugs/penicillins?query=penicillin%20allergic. Accessed Oct. 24, 2019.
  9. Cephalosporins. Merck Manual Professional Version. https://www.merckmanuals.com/professional/infectious-diseases/bacteria-and-antibacterial-drugs/cephalosporins?query=cephalosporins#. Accessed Oct. 24, 2019.

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