Patient education: Penicillin allergy (The Basics)

What are penicillins? — Penicillins are a group of related antibiotics. Antibiotics are medicines used to treat bacterial infections.

There are lots of different penicillins. They include penicillin G, amoxicillin, ampicillin, and other drugs that end in "cillin." If you have an allergy to 1 of these medicines, you should assume that you are allergic to all penicillins.

What is a penicillin allergy? — A penicillin allergy is a bad reaction to penicillin or a related drug. It happens when the body's immune system responds to the drug as if it were a dangerous invader and tries to fight it off.

The immune system's job is to fight off infection. Normally, it should not react to a drug as though it were an invader. But this is what happens when a person has a drug allergy.

An allergic reaction can be:

●Immediate – This type of reaction happens quickly after taking penicillin or a related drug, usually within minutes to 1 hour.

●Delayed – This type of reaction usually happens after more than 1 dose of the drug, often after taking the drug for days.

A drug allergy is not the same as a drug side effect. Side effects are unintended or unwanted symptoms that can happen after taking a drug. Examples might include mild nausea or diarrhea. Sometimes, people have side effects and think that they are having an allergic reaction. Tests can show if a person has a true penicillin allergy.

Penicillin allergy can happen at any age. It is more likely to happen in people who need to take penicillin drugs frequently.

What are the symptoms of an allergic reaction? — It depends on whether the reaction is immediate or delayed.

With an immediate reaction, symptoms can include:

●Hives, which are raised patches of skin that are usually very itchy (picture 1)

●Itchy skin

●Flushing, which is when your skin turns red and feels hot

●Swelling of the face, hands, feet, or throat

●Throat tightness, hoarse voice, wheezing, or trouble breathing

●Nausea, vomiting, or belly pain

●Feeling lightheaded

This type of allergy is serious because it can get worse if you keep taking the drug. It can turn into a life-threatening allergic reaction called "anaphylaxis."

With a delayed reaction, the main symptom is a rash that begins after a few days of taking a drug. The rash often spreads over much of the skin (picture 2). Sometimes, it is itchy, but not always. This type of drug reaction does not involve swelling, trouble breathing, throat tightness, or the other symptoms listed above. The rash doesn't usually get worse or affect anything besides the skin. (A rare but severe skin reaction to a drug, called Stevens-Johnson syndrome, is different. This type of reaction requires immediate medical attention.)

How are allergic reactions treated? — The first step is to stop taking the medicine that caused the reaction.

Doctors can treat severe allergic reactions with a medicine called "epinephrine." Then, they will try to figure out what caused the reaction and whether to switch you to another antibiotic.

Mild delayed reactions, like a rash, go away on their own once the drug is stopped.

Is there a test for penicillin allergy? — Yes. Your doctor or nurse will ask you about your symptoms and whether you have had a reaction to a penicillin in the past. If they think that you might have a penicillin allergy, they will probably send you to an allergist (allergy doctor) for testing.

Tests might include:

●Skin tests – For this test, the doctor puts a drop of the drug you might be allergic to on your skin and makes a tiny prick in your skin. Or they might inject a very tiny amount of the drug. Then, they watch your skin to see if it turns red or bumpy.

●"Drug challenge" test – For this test, you take a small amount of a drug while the doctor watches to see if you have a reaction. If you do have a reaction, they will be ready to treat it right away. If you do not, they can slowly increase the dose. Drug challenge testing is usually only done if the doctor thinks that you are not likely to have a serious allergy.

Sometimes, a person thinks that they are allergic to penicillin, but they actually are not. This could be because they had side effects that they thought were symptoms of an allergic reaction. Or they might have had an allergy in the past, but it went away.

If you think that you might have an allergy, get testing to find out for sure. That's because penicillin and related drugs are used to treat many common infections, and sometimes other medicines do not work as well. So you should only avoid penicillins if you are truly allergic.

How can I prevent an allergic reaction? — If you know that you have a penicillin allergy that causes a severe or immediate reaction, you should completely avoid penicillin and all related drugs. Most of the time, other antibiotics can be used instead.

Certain infections can only be treated with penicillin drugs. If you get one of these infections and you need treatment, your doctor might recommend something called "desensitization." This involves giving you the drug in a controlled way, starting with very small amounts. It can cause your body to tolerate the drug for a short time without having a reaction.

Desensitization should only be done by an allergist in a hospital or clinic. It does not cure an allergy, but can allow you to take penicillin for a short time if you really need to.

If you had a mild delayed reaction in the past, you might still be able to take penicillins. But your allergist will want to monitor you to make sure that you do not have a more serious reaction.

What else should I know if I have a penicillin allergy?

●Make sure that you know exactly what drug or drugs you are allergic to. Medicines sometimes have more than 1 ingredient and go by more than 1 name, which can be confusing. For example, "Augmentin" is a brand name for amoxicillin and clavulanate. Because amoxicillin is a penicillin, this medicine is not safe for people with a penicillin allergy.

●Tell all of your doctors and nurses about your allergy. Tell your pharmacy, too.

●Wear a medical alert bracelet or necklace explaining your drug allergy.

When should I call the doctor? — That depends on what kind of reaction you have.

Call for an ambulance (in the US and Canada, call 9-1-1) if you start a new medicine and have any of these symptoms:

●Wheezing or trouble breathing

●Chest tightness or pain

●Fainting or feeling like you might faint

●Swelling of your lips, tongue, or throat that makes it hard to breathe

Call your doctor or nurse if you start a new medicine and have any of these symptoms:

●Hives

●Swelling of the face

●High fever

●Painful skin

●Skin blisters

●Pain and irritation of the pink, moist tissue that lines the eyes, mouth, vagina, and other organs

More on this topic

Patient education: Drug allergy (The Basics)
Patient education: Allergy skin testing (The Basics)
Patient education: Anaphylaxis (The Basics)
Patient education: Side effects from medicines (The Basics)
Patient education: Stevens-Johnson syndrome and toxic epidermal necrolysis (The Basics)
Patient education: What you should know about antibiotics (The Basics)

Patient education: Allergy to penicillin and related antibiotics (Beyond the Basics)
Patient education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)
Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)