Penicillin allergy is a common drug allergy that occurs when the immune system reacts abnormally to penicillin or related antibiotics, such as amoxicillin and cephalosporins. Symptoms of penicillin allergy can range from mild to severe and can occur within minutes to hours after exposure to the drug.

Mild symptoms may include:

• Rash or hives
• Itchy skin
• Runny or stuffy nose
• Watery eyes
• Nausea or vomiting
• Diarrhea

Severe symptoms may include:

• Swelling of the face, lips, tongue, or throat
• Difficulty breathing
• Rapid heartbeat
• Low blood pressure
• Anaphylaxis (a life-threatening allergic reaction)

If a person experiences any symptoms of an allergic reaction after taking penicillin or a related antibiotic, they should seek medical attention immediately.

Diagnosis of penicillin allergy involves a combination of medical history, physical exam, and allergy testing. Skin tests, blood tests, and oral challenges may be used to determine if a person is allergic to penicillin or related antibiotics.

If a person is diagnosed with penicillin allergy, they should avoid penicillin and related antibiotics, and use alternative antibiotics when necessary. If a person is unsure if they are allergic to penicillin, they should consult with their healthcare provider or an allergist to determine if allergy testing is necessary.

It is important to note that having a penicillin allergy does not necessarily mean a person is allergic to all antibiotics or all classes of antibiotics. A healthcare provider can help determine safe and effective alternative antibiotics for individuals with penicillin allergy.

Penicillin allergy occurs when your immune system becomes hypersensitive to the drug — mistakenly reacting to the drug as a harmful substance, as if it were a viral or bacterial infection.

Before the immune system can become sensitive to penicillin, you have to be exposed to the medication at least once. If and when your immune system misidentifies penicillin as a harmful substance, it develops an antibody to the drug.

The next time you take the drug, these specific antibodies flag it and direct immune system attacks on the substance. Chemicals released by this activity cause the signs and symptoms associated with an allergic reaction.

Previous exposure to penicillin may not be obvious. Some evidence suggests that trace amounts of it in the food supply may be sufficient for a person's immune system to create an antibody to it.

Penicillins and related drugs

Penicillins belong to a class of antibacterial drugs called beta-lactam antibiotics. Although the mechanisms of the drugs vary, generally they fight infections by attacking the walls of bacterial cells. In addition to penicillins, other beta-lactams more commonly associated with allergic reactions are a group called cephalosporins.

If you've had an allergic reaction to one type of penicillin, you may be — but are not necessarily — allergic to other types of penicillin or to some cephalosporins.

Penicillins include:

• Amoxicillin
• Ampicillin
• Dicloxacillin
• Nafcillin
• Oxacillin
• Penicillin G
• Penicillin V
• Piperacillin
• Ticarcillin

Cephalosporins include:

• Cefaclor
• Cefadroxil
• Cefazolin
• Cefdinir
• Cefepime (Maxipine)
• Cefotetan
• Cefprozil
• Cefuroxime
• Cephalexin (Keflex)

If you have a penicillin allergy, the simplest prevention is to avoid the drug. Steps you can take to protect yourself include the following:

• Inform health care workers. Be sure that your penicillin allergy or other antibiotic allergy is clearly identified in your medical records. Inform other health care professionals, such as your dentist or any medical specialist.
• Wear a bracelet. Wear a medical alert bracelet that identifies your drug allergy. This information can ensure proper treatment in an emergency.

Interventions for a penicillin allergy can be divided into two general strategies:

• Treatment for the current allergy symptoms
• Desensitization to penicillin

Treating current symptoms

The following interventions may be used to treat the symptoms of an allergic reaction to penicillin:

• Withdrawal of the drug. If your doctor determines that you have a penicillin allergy — or likely allergy — discontinuing the drug is the first step in treatment.
• Antihistamines. Your doctor may prescribe an antihistamine or recommend an over-the-counter antihistamine such as diphenhydramine (Benadryl) that can block immune system chemicals activated during an allergic reaction.
• Corticosteroids. Either oral or injected corticosteroids may be used to treat inflammation associated with more-serious reactions.
• Treatment of anaphylaxis. Anaphylaxis requires an immediate epinephrine injection as well as hospital care to maintain blood pressure and support breathing.

Drug desensitization

If there are no other suitable antibiotic treatment options available, your doctor may recommend a treatment called drug desensitization that may enable you to take a course of penicillin to treat an infection. With this treatment, you receive a very small dose and then progressively larger doses every 15 to 30 minutes over the course of several hours or a few days. If you can reach the desired dosage with no reaction, then you can continue the treatment.

It's important to take the drug as directed to maintain your tolerance to it during the entire course of treatment. If you need penicillin in the future, you will need to repeat the desensitization treatment.

You're carefully monitored during the intervention, and supportive care is available to treat reactions. Desensitization is not always successful, and there is a risk of serious reactions.

Preparing for your appointment

Be prepared to answer the following questions. These details will be important in helping your doctor determine the cause of your symptoms.

• What symptoms did you experience?
• What is the name of the penicillin or other antibiotic you were taking?
• Why were you prescribed the drug?
• Have you had these symptoms in the past when you weren't taking this drug?
• How long after taking penicillin did the symptoms begin?
• How long did the symptoms last?
• Have you stopped taking the drug?
• What other drugs, herbal medications, vitamins or other dietary supplements do you take?
• At what time of day do you take your other medications or supplements?
• Have you increased the dosage of any regular drug or supplement?
• Have you stopped taking your regular medications or supplements?
• Did you take anything to treat your symptoms, and what was the effect?
• Have you had a reaction to any drug in the past? If so, what drug was it?
• Do you have hay fever, food allergy or other allergies?
• Is there a history of penicillin or other drug allergies in your family?

You may want to take pictures of any condition, such as a rash or swelling, to show your doctor. These may help your doctor if symptoms have subsided by the time of your appointment.