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Five things you need to know about penicillin allergy

Physicians from McMaster University in Canada recently published a list of facts about penicillin allergy the public may not know.

Almost a century has passed since the discovery of the first antibiotic. Nowadays, antibiotics are among the most prescribed drugs.

Among them, penicillins are the preferred choice to treat infections such as pneumonia, meningitis, and syphilis.

However, there are a significant amount of people who have an adverse reaction to this type of antibiotic; here are some facts about penicillin allergy that you may not already know.

What is penicillin allergy?

While these antibiotics are effective against bacterial infections, they may cause adverse reactions, like many other drugs. Up to 10% of patients may experience adverse reactions to penicillin.

Adverse reactions to penicillins can be of four different types (types I-IV). Of these four categories, only a type-I hypersensitivity can be considered a true allergy. During a type-I reaction, the body produces specific antibodies called IgE and develops inflammation.

The results of a type-I reaction are symptoms such as hives, swelling, respiratory symptoms, and life-threatening anaphylaxis in more severe cases.

Symptoms usually appear within a few minutes to a few hours after exposure to penicillin. Reactions of type II, III, and IV appear delayed, usually after days of exposure to the antibiotics.

The appearance of symptoms might hurry patients to consider themselves allergic to these drugs.

Penicillins trigger different reactions in the human body, and some could be intolerances rather than true allergies. For this reason, a medical evaluation could be necessary.

Physicians from McMaster University in Ontario, Canada, recently published a list of facts about penicillin allergy that the public may not know.

The document is published in the Canadian Medical Association Journal (CMAJ).

1. Penicillin allergy is reported frequently, but most patients can tolerate these antibiotics

About 10% of patients report an allergy. Among these, 90 to 95% do not suffer from a true allergy. The reasons for this discrepancy include the wrong identification of intolerances as allergies and the fact that these allergies weaken with time.

2. Penicillin allergy may resolve with time

Penicillin allergy weakens and disappears in half of the patients over five years, and in 80% of patients over ten years. Individuals who had reactions over ten years ago are now unlikely to develop an allergic reaction.

3. The label of penicillin allergy is negative for patients and healthcare systems

The label of penicillin allergy is associated with the use of second-line and broad-spectrum antibiotics, which tend to be more expensive and less effective than penicillin.

In addition, the use of these antibiotics increases the risk of infections with dangerous antibiotic-resistant bacteria such as Staphylococcus aureus and Clostridium difficile.

4. A medical evaluation is necessary to assess risks

Patients who experience a common side effect of this antibiotic, such as nausea, should not conclude that they are allergic. Similarly, a family history of penicillin allergy alone does not justify penicillin avoidance in patients with no history of reactions.

On the contrary, patients who experience severe symptoms should avoid taking these antibiotics. A medical evaluation by a specialist is necessary to assess whether this is a true allergy, especially when adverse reactions are unclear.

5. Testing is safe and accurate, but underused

Allergy referral and testing performed by trained personnel are safe, cost-effective, and accurate in almost 100% of cases in both adults and children. Typical penicillin allergy tests include skin tests and drug challenges.

Patients concerned about being allergic to penicillin should talk to a doctor and get advice on allergy testing.

Written by Raffaele Camasta, PhD


  1. McCullagh, D. J., & Chu, D. K. (2019). Penicillin allergy. CMAJ : Canadian Medical Association Journal = Journal de l’Association Medicale Canadienne, 191(8), E231.
  2. You probably don’t have a penicillin allergy.
  3. Shenoy, E. S., Macy, E., Rowe, T., & Blumenthal, K. G. (2019). Evaluation and Management of Penicillin Allergy. JAMA, 321(2), 188-199.
  4. Chang, C., Mahmood, M. M., Teuber, S. S., & Gershwin, M. E. (2012). Overview of Penicillin Allergy. Clinical Reviews in Allergy & Immunology, 43(1–2), 84–97.


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