Locations:
Search IconSearch

Think You’re Allergic to Penicillin? Why You May Be Mistaken

A simple skin test can tell you for sure

Hand holding two blue pills

When your doctor asks if you’re allergic to penicillin, do you say yes? Lots of people do.

Advertisement

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Penicillin is a commonly-prescribed antibiotic that helps fight off a variety of infections caused by bacteria — from ear aches and sinus infections to pneumonia and meningitis. For a few people, however, it causes an allergic reaction that may include hives, rashes, itchy eyes, fever, shortness of breath, and swollen lips, tongue or face.

About 10% of the population reports a penicillin allergy, but less than 1 in 10 of those people are truly allergic. Moreover, about 80% of those who were once allergic to the drug lose their sensitivity to penicillin after 10 years. So why do so many people think they’re allergic when they’re not? And how can you know for sure?

“Penicillin is the most frequent allergy reported by patients seen at Cleveland Clinic,” says allergy and immunology specialist David Lang, MD. And that is sometimes a problem.

“When a doctor sees you have a label of penicillin allergy, it is often a barrier for getting the most appropriate care in terms of antibiotic selection,” he says.

Reasons you might think you’re allergic (when you’re not)

There are two primary reasons why you may mistakenly think you’re allergic.

1. You had an allergic reaction to something else. You may have noticed a reaction in the past while taking a penicillin-type antibiotic that was not actually caused by the penicillin.

Advertisement

For example, a doctor may give antibiotics for an upper-respiratory infection. Sometimes an infection, particularly in children, also causes a rash, Dr. Lang says. Once you experience symptoms, you get an antibiotic, a rash occurs, and you mistakenly think the penicillin caused the rash.

2. You had other adverse side effects. An adverse reaction to penicillin may lead you to mistakenly believe that you are allergic. For example, penicillin may upset your stomach. While this is a common side effect, it does not mean you’re allergic.

“There are also times when people have coincidental adverse experiences,” says Dr. Lang. For example, you may have a series of headaches or unpleasant symptoms while you are taking the antibiotic, but something else is causing those problems.

Is there a downside to avoiding penicillin?

Yes, says Dr. Lang. You may miss out on the most effective treatment for your condition.

Your doctor must find alternative antibiotics if you have a penicillin allergy. And alternative antibiotics often:

  • Are more costly.
  • Pose a greater risk for adverse effects.
  • Are less effective.

“A penicillin allergy needs to be challenged,” he says. “When we perform a formal allergy evaluation with penicillin skin testing, the results are negative in about every 9 out of 10 patients we evaluate. The negative skin testing implies that penicillin can be taken without an increased risk for allergic reaction compared with the general population.”

How does the testing work?

Skin testing for a penicillin allergy is commonly performed and can be done safely — even in patients who are critically ill or who have had serious reactions to penicillin, Dr. Lang says.

1.Validating the test

To ensure that your skin reacts normally, a nurse will apply both a saline solution and a histamine solution. If your skin reacts to the saline, that means your skin is too sensitive and the test cannot be interpreted.

The histamine solution should cause a red, raised, itchy area, similar to a mosquito bite. If there is no reaction, your skin test may not show an allergy even if you have one, and for this reason the test cannot be interpreted.

If your skin does not react to the saline, but reacts to the histamine, this validates the skin test: Your skin has the capacity to react to a potential allergen.

2. Testing for penicillin allergy

At the same time, a nurse places liquid drops of penicillin on your skin and scratches through the liquid so that it gets underneath the top layer of your skin. A reaction that is similar to a mosquito bite should occur in minutes if you are allergic to penicillin.

If your skin does not react to penicillin, an intradermal test is the next step, which involves a nurse injecting a small amount of penicillin liquids into your skin. After 15 minutes, the skin test is examined for signs of an allergic reaction.

Advertisement

3. Applying a challenge dose

If both the skin-prick and intradermal skin tests are negative, you’ll receive a challenge dose of penicillin or another penicillin-type drug (such as amoxicillin), and then you will be observed for 30 minutes. This challenge dose is important, Dr. Lang says, as it validates the skin testing and demonstrates that you can take penicillin and penicillin-like drugs without an allergic reaction.

Advertisement

Learn more about our editorial process.

Related Articles

Hives on a person's arm from an allergic reaction to medication
June 25, 2025/Health Conditions
Salicylate Sensitivity: Symptoms and Management

If you’re sensitive to aspirin or other salicylates, limiting exposure is the best remedy

X-ray of implanted pacemaker
July 11, 2025/Heart Health
How To Know if You Need a Pacemaker

Fainting, heart palpitations and shortness of breath are just a few signs your heart may need help

Person sitting on floor against sofa with head on arms resting on knees
July 11, 2025/Mental Health
Understanding Situational Depression

Major life events and trying times can trigger reactive depression symptoms, like prolonged sadness, irritability and hopelessness

Adult inserting clear dental aligner tray into mouth
July 10, 2025/Oral Health
Can Adults Get Braces?

You can correct dental issues no matter your age

Kitchen sponge sitting on edge of dirty sink filled with dirty pans
July 10, 2025/Infectious Disease
What Is Cross Contamination and How Can You Avoid It?

Washing your hands, thoroughly cleaning kitchen tools and preparing meat separately can reduce the risk of foodborne illness

Person crying, with red, swollen eyes, staring straight at the viewer
July 9, 2025/Eye Care
How To Get Rid of Swollen Eyes From Crying

Be patient and try at-home remedies like applying a cold compress and elevating your head while sleeping

Couple holding hands on one side of desk, with doctor on the other side
How Congenital Adrenal Hyperplasia Can Affect Fertility

Hormonal imbalances can make it harder to conceive when you’re living with CAH, but treatments are available

Tired man with head in hand, eyes closed
July 9, 2025/Allergies
Tired? You May Be Experiencing Allergy Fatigue

Drowsiness is a side effect of inflammation, disrupted sleep and, sometimes, your allergy medicine

Trending Topics

Person in bed sleeping on their side, covers off
Breathing Problems? Try These Sleep Positions

If you’re feeling short of breath, sleep can be tough — propping yourself up or sleeping on your side may help

A couple looking at skyline, with one person slightly behind the other, head bent down
What Is Anxious Attachment Style — and Do You Have It?

If you fear the unknown or find yourself needing reassurance often, you may identify with this attachment style

Glasses and bottle of yellow-colored prebiotic soda, with mint, lemon and ginger garnish
Are Prebiotic Sodas Good for You?

If you’re looking to boost your gut health, it’s better to get fiber from whole foods

Ad