How to Tell If You’re Having an Allergic Reaction to an Antibiotic: Pictures and Symptoms

You’ve probably taken an antibiotic at some point in your life. They are a safe and effective way to treat many bacterial infections. But sometimes people experience a side effect from an antibiotic. Does this mean they’re allergic to it?

While antibiotics can cause allergic reactions, they actually aren’t that common. Rashes and other side effects, like diarrhea, are much more common. But since allergies can cause rashes and diarrhea, how can you tell the difference between an antibiotic side effect and a true allergic reaction? 

What’s the difference between a drug allergy, drug reaction, and  drug rash?

A drug reaction is the term for any unwanted or unpleasant effect that happens from taking a medication. It includes drug rashes and drug allergies. 

Here’s a closer look at the different types of drug reactions:

• A drug allergy is when your immune system overreacts to a medication. True drug allergies are much less common than people think. In fact, only 5% to 10% of drug reactions are from an allergy.

• A drug rash is any skin breakout a medication causes. It’s usually from some type of allergic reaction, but it doesn’t have to be.   

• A side effect is an unwanted outcome associated with a medication. Different factors make it more likely to have a side effect, like your age and if you have other health problems. Most drug reactions are from side effects.

Drug reactions can be mild or severe, and they can even be life-threatening. Any type of medication — including prescriptions, over-the-counter medications, and supplements — can cause a drug reaction.

Typical reactions to antibiotics

Antibiotics can cause different types of drug reactions. They can cause side effects like: 

• Nausea

• Vomiting

• Indigestion

• Diarrhea 

Sometimes antibiotics can cause an allergic reaction. This is when the immune system reacts to the antibiotic. Typically, allergies to antibiotics cause: 

• Rash

• Itching

• Mouth swelling

• Breathing problems   

EXPERT PICKS: WHAT TO READ NEXT

• Drug rashes: Learn how to identify the different types of skin rashes caused by antibiotics and other medications (with pictures). 

• Penicillin allergy: Learn how to know if you’re allergic to this common antibiotic and which medications to take instead. 

• What causes hives? Medications aren’t the only culprit. From viruses to stress, learn the most common causes of hives. 

Allergic reactions to antibiotics

For some people, taking a certain antibiotic causes their immune system to act out against itself. This happens because the immune system thinks the antibiotic is an “invader.” It’s a simple case of mistaken identity, but it can cause chaos. 

There are two main types of allergic reaction. Immediate allergic reaction is the most common, so we’ll cover it first.

Immediate allergic reaction (antibody mediated)

When people talk about an allergic reaction, they’re usually referring to this type of reaction. The immune system launches an immediate defense with antibodies (specifically IgE antibodies). These allergy symptoms come on within minutes to hours of taking the medication: 

• Hives or itchy skin

• Swelling under the skin (angioedema)

• Coughing, wheezing, or difficulty breathing

• Diarrhea or vomiting

• Anaphylaxis (a severe allergic reaction that needs immediate medical attention)

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Any medication can cause this type of reaction, but certain antibiotics (like penicillin and amoxicillin) are more likely to. Even though this is the most common type of antibiotic allergy, keep in mind that this type of allergy is still very rare. Less than 1% of the population is truly allergic to penicillin.

Delayed allergic reaction (T-cell mediated)

Another type of allergic reaction can happen days to weeks after taking a new medication. This involves a different part of the immune system (the T cells). This type of reaction usually causes an itchy skin rash that can be mild or severe. 

Severe reactions may also affect other organs, like the liver or kidneys. Examples of these types of serious drug allergies include:

• Drug reaction with eosinophilia and systemic symptoms (DRESS)

• Stevens-Johnson syndrome and toxic epidermal necrolysis (SJS/TEN)

Severe drug allergies of this type may cause skin blisters and sores on your eyes, genitals, or in your mouth.

Pictures of antibiotic rashes and allergies

Here are some pictures of different antibiotic rashes on different skin tones. 

A person develops a rash on the torso after taking an antibiotic like penicillin.

An antibiotic drug eruption appears on the leg.

A rash triggered by an antibiotic appears on a person’s back.

A person’s back is covered in hives, which are caused by medications like antibiotics.

What are the most common side effects of antibiotics?

Antibiotics can also cause many side effects that aren’t related to allergic reactions. Here are some common antibiotic side effects:

• Nausea or vomiting

• Diarrhea

• Indigestion or feeling bloated

• Yeast infections

• Headache

• Dizziness

• Nasal congestion and runny nose

• Muscle pains

The most common side effects are nausea, vomiting, diarrhea, and indigestion. About 10% of people get gastrointestinal side effects with antibiotics.

If you experience an antibiotic side effect, it doesn’t always mean you have to stop taking the medication. Sometimes treating the symptoms or adjusting the dose can help. Don’t stop taking antibiotics without talking to your prescriber first. 

Does a drug rash always mean an allergy?

No, not necessarily. The cause of a drug rash is often some type of allergy. But that’s not always the case. For example, people can get a rash with amoxicillin treatment when they have mononucleosis (caused by the Epstein-Barr virus). This can be alarming, but an allergy isn’t the cause. And just a reminder: Antibiotics don’t work against viruses.

Some types of drug rashes can mean you’re having a more serious drug allergy. If you’re experiencing any of these symptoms, it’s important to get immediate medical attention:

• The rash starts to hurt.

• The skin blisters or forms pus, bumps, or pimples.

• Sores develop in your mouth, eyes, or genitals.

• You develop coughing, wheezing, or difficulty breathing.

If you develop a rash after starting a new antibiotic, it’s a good idea to stop taking it and notify a healthcare professional.

Do you have a penicillin allergy?

If you’ve been told you have a penicillin allergy, chances are you probably don’t. About 10% of people report having a penicillin allergy, but more than 90% of them aren’t actually allergic. How does this happen? There are a few possible reasons:

• The reaction you experienced was a side effect of penicillin (like diarrhea) rather than a true allergy.

• Your symptoms were from something else, like a virus or another medication you were taking.

• You grew out of it — over 90% of people with a real penicillin allergy don’t have it after 10 years.

Knowing if you have a true penicillin allergy is important. This is because penicillins (which include many related antibiotics) are very good at treating many different infections. If you can’t take them, you may have to get a different antibiotic that causes more side effects. 

Diagnosing penicillin allergies

It’s possible to figure out if you’re really allergic to penicillin. Specially trained physicians (an allergist or immunologist) use a skin prick test, where they scrape a small amount of medication into your skin. 

If you don’t develop an itchy skin spot (like a bug bite), you’re probably not allergic. After this, you may take one dose of the medication to confirm the results. This process is very safe because you do it in a controlled setting of the doctor’s office. 

If you think you have a penicillin allergy, talk with a healthcare professional to see if you should be tested to confirm the allergy.  

How do you treat an allergic reaction to antibiotics?

Many antibiotic reactions will go away just by stopping the medication. For milder allergic reactions to antibiotics, some treatments can help. For example, antihistamines may help treat itching and hives. If you’re experiencing a severe allergic reaction to an antibiotic (like difficulty breathing), it’s an emergency and you should get immediate medical care. In these cases, treatment often includes epinephrine.

If you ever develop a new or bothersome reaction to a medication, contact a healthcare professional to discuss your symptoms. Depending on the reaction and what’s being treated, they may recommend one of the following:

• Stop taking the medication, and switch to something else.

• Change the medication dose.

• Continue taking the medication while treating the reaction.

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Written by:

Maria Robinson, MD, MBA

Maria Robinson, MD, MBA, is a board-certified dermatologist and dermatopathologist who has practiced dermatology and dermatopathology for over 10 years across private practice, academic, and telehealth settings. She is a fellow of the American Academy of Dermatology and the American Society of Dermatopathology.

Edited by:

Karla Robinson, MD

Karla Robinson, MD, is a medical editor for GoodRx. She is a licensed, board-certified family physician with almost 20 years of experience in health through varied clinical, administrative, and educational roles.

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