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Thomas Lupoli, D.O.

Everything You Need to Know About Penicillin Allergies

penicillin allergy treatment jacksonville fl

Updated Sept 2022

Alexander Fleming first stumbled on penicillin as he grew bacteria in culture dishes in his London lab during September of 1928. Mold accidentally grew in the dishes and he noticed the mold had developed clear areas around itself where the bacteria could not grow. His discovery, and the subsequent scientific initiatives that allowed penicillin to be made widely available, is known as one of the greatest advancements in modern medicine.

Before the availability of penicillin, many bacterial infections were simply untreatable. People with life-threatening blood poisoning from a simple cut would be cared for in a hospital with little more than a “wait and see” approach. Today, penicillin comes in many forms and is still widely used to combat a range of bacterial infections. But in recent years, more and more patients have reported symptoms of penicillin allergies. In fact, according to the CDC, some 10% of patients in the U.S. report having had an allergic reaction to penicillin.

Read on as we explore how and why people develop allergies, proper identification, and what you can do to treat and prevent reactions.

What Patients Should Know About Penicillin Allergies

As stated above, as many as 1 in 10 Americans reported having an allergic reaction to one of the forms of penicillin (i.e. amoxicillin, dicloxacillin, penicillin G and V, among others). However, the CDC and other leading research institutions like the Mayo Clinic, warn that many of these reported incidents aren’t actually penicillin allergies at all.

Further evaluation of the 10% of the population who reported having penicillin allergies showed that 90% of these individuals were not truly allergic to a form of penicillin. For the 1% of the population that is truly allergic, as many as 80% of these patients have dramatic decreases in sensitivity to the antibiotic 10 years later.

If so many patients aren’t really allergic, what is the problem? The problem begins when patients mistake certain side effects or symptoms as an allergy to penicillin, reporting this to their doctor, and their doctor prescribing a broad-spectrum antibiotic as an alternative.The problem with this approach is threefold:

  1. You are receiving a non-targeted treatment plan, what’s known medically as “suboptimal antibiotic therapy”. As opposed to a narrow-spectrum antibiotic like penicillin, the broad spectrum antibiotic wasn’t designed for the specific infection your body’s fighting, so it isn’t nearly as effective at treating it.
  2. Broad spectrum antibiotics are generally more costly than penicillin, resulting in higher healthcare costs.
  3. The prolific use of broad spectrum antibiotics is contributing to the rise of antibiotic resistance. Antibiotic resistant infections cause millions of illnesses and thousands of deaths each year.

The other side of this coin is not taking a patient seriously who does in fact have a penicillin allergy. We’ll next explore several symptoms to watch out for if you’ve taken penicillin and had a reaction.

Symptoms of a Penicillin Allergy

An allergic reaction is when your immune system mistakenly thinks the drug is a harmful substance. After an initial exposure to the drug, your body develops antibodies to the substance. When re-exposed, chemicals flood the body which causes the signs and symptoms of an allergic reaction. For the vast majority of cases, an allergic reaction to penicillin will occur within an hour of taking the drug. However, sometimes the reactions can manifest as much as several weeks after exposure. Signs to look out for that you’re having an allergic reaction to penicillin include:

  • A rash, hives, or itchy skin
  • Running a fever
  • Swelling
  • Any kind of wheezing/shortness of breath
  • A runny nose or itchy/watery eyes
  • Anaphylaxis

Although rare, anaphylaxis is the most serious allergic reaction that can occur. When this happens, the body’s systems fail to function properly which can cause immediate life-threatening symptoms such as trouble breathing, seizures, or loss of consciousness. If you or a loved one has taken penicillin and exhibits any kind of difficulty breathing, you need to call an ambulance or get them to an emergency room as quickly as possible.

Diagnosis of a Penicillin Allergy

There are several paths for doctors to take when diagnosing penicillin allergies. The best course is a combination of three methods: history and physical exam, a skin test, and a challenge dose.

During a history and physical exam, your doctor should ask some crucial questions to help them understand the nature of the reaction such as the specific medication(s) you were taking when the reaction occurred, what type of reaction occurred, how long into the course did the reaction start, how long did the reaction last, how did you treat the reaction, and what was the final outcome. If your doctor doubts a penicillin allergy after this examination, they may recommend further testing and refer you to an allergist for allergy testing.

An allergy skin test usually involves putting a small amount of the suspected allergen (penicillin) on the skin of your arm and waiting to see if a small skin reaction occurs. The allergist will typically use a toothpick-like tool called a stylet to apply the substance to the test area. Within about 15 minutes of applying the test, a raised bump resembling a mosquito bite will appear on the skin if you are truly allergic to penicillin. If no reaction (bump) appears, then that would indicate about 97% certainty you are not allergic. However to make completely certain you are not allergic to penicillin, one additional test is usually performed.

Graded drug challenge are considered the gold-standard for definitively excluding or confirming the presence of a drug allergy. This usually involves administering several incremental doses of amoxicillin under the direct supervision of the allergist. This test is only performed when there is no evidence for a true penicillin allergy.

Treatment for Penicillin Allergies

The somewhat obvious first step for someone suffering from a penicillin allergy is to avoid the antibiotic altogether. To treat the immediate symptoms, your doctor may prescribe a strong antihistamine or you can take an over the counter variety. More adverse reactions may require the use of corticosteroids which are either taken orally or injected by a physician. In the most extreme reactions, medical personnel will inject a patient with epinephrine to counteract the symptoms of anaphylaxis.

Frequently Asked Questions About Penicillin and Penicillin-like Allergies:

Is Penicillin a sulfa drug?

No, Penicillin and related drugs do not contain sulfa. Septra and Bactrim are sulfa containing antibiotics.

How do you know if you’re allergic to penicillin?

You will exhibit any number of different allergic reactions such as a rash or hives, wheezing/shortness of breath, or in extreme cases may have dizziness, vomiting, seizures, or loss of consciousness. AASNF’s medical staff can quickly provide testing services that will determine with certainty if your allergy symptoms are related to penicillin.

What happens if you’re allergic to penicillin?

If you’re truly allergic to penicillin, you will experience allergic reactions as your immune system attempts to combat what it’s perceived as a threat to your body. You’ll usually have a rash or itchy skin, a runny nose, and itchy or watery eyes. When the allergy has been verified by a medical professional, a normal course of treatment for the symptoms includes over-the-counter antihistamines or prescription of corticosteroids. Generally, avoidance of penicillin is the first step in treatment.

When should you go to the ER for an allergic reaction?

Anaphylaxis is a serious, life-threatening reaction to an allergic substance. It can come in many forms but any kind of wheezing/shortness of breath, irregular blood pressure, loss of consciousness, or vomiting could be a sign that an anaphylactic response is occurring and you should call an ambulance or get to an ER as quickly as possible.

How long does an allergic reaction to penicillin last?

Everyone’s body chemistry is different and the allergic reaction to penicillin will vary from person to person. However, even with treatment, the signs and symptoms of an allergic reaction to penicillin can last anywhere from 2 to 4 weeks.

Can you outgrow an allergy to penicillin?

Yes. Of the estimated 1% of the population that truly has an allergy to penicillin, as many as 80% of these individuals report a dramatic decrease in their symptoms 10 years after the first occurrence.

Can you take Bactrim or Septra if allergic to penicillin?

Yes, providing that you are not allergic to Bactrim/Septra of course. Bactim/Septra are sulfa-containing antibiotics and do not contain the same chemical compounds as penicillin. As with any treatment, always consult your doctor before trying a new course or if you suspect you may be allergic to a certain medication.

National Penicillin Allergy Day is an annual event that occurs every September 28th, in honor of penicillin’s discovery by Alexander Fleming in September of 1928. The day seeks to bring awareness to correctly identifying penicillin allergies and the proper course for those that really do have an allergy. For anyone suspecting they have an allergy to penicillin, the highly-trained and knowledgeable staff of Allergy and Asthma Specialists of North Florida can quickly test you to properly diagnose what’s causing your signs and symptoms. We have four offices conveniently located around Jacksonville to serve you. Call 904-730-4870 to request an appointment or make an online appointment today.


American Academy of Allergy Asthma & Immunology, “Safety of test doses for the evaluation of adverse drug reactions”.
American Chemistry Society, “Discovery and Development of Penicillin
American College of Allergy Asthma & Immunology, “Drug Allergies
Centers for Disease Control and Prevention, “Is it Really a Penicillin Allergy?”
Children’s Hospital of Philadelphia, “Preventing Antibiotic Resistance and the Spread of Superbugs
Mayo Clinic, “Allergies
Mayo Clinic, “Allergy Skin Tests
Mayo Clinic, “Hives and Angioedema
Mayo Clinic, “Penicillin Allergy
WebMD, “Skin Allergy Types and Triggers