Penicillin allergy

A penicillin allergy is one of the most common drug allergies. Penicillin belongs to the drug family known as beta-lactam antibiotics and the most commonly prescribed antibiotics in this family are ampicillin and amoxicillin. These antibiotics are relatively inexpensive and generally effective in the treatment of many common bacterial infections. These include skin, ear, sinus and upper respiratory infections. Penicillin is not effective against viruses.

Penicillin allergies develop largely because the immune system responds to the drug as if it were a harmful substance instead of a helpful remedy. An individual is not born allergic to penicillin. An allergy to the drug will develop only once an individual has been exposed to it. Re-exposure to penicillin or related antibiotics at a later stage can then trigger an allergic reaction. Penicillin allergies are most commonly found in young adults, however reactions can occur at any age.

Symptoms of an allergic reaction to penicillin may include:

• Hives
• Below-the-skin swelling
• Asthma symptoms
• Itchy eyes
• Swollen lips – tongue and face

In some instances you may suffer with an anaphylactic shock which may include difficulty breathing, hives, wheezing, dizziness, loss of consciousness, rapid or weak pulse, skin turning blue, diarrhea, nausea, and vomiting. This will usually occur about 5 minutes to 2 hours after you take the penicillin. Other reactions can occur up to 2 to 3 days after taking the penicillin and in some cases there can even be a delayed reaction several weeks later.

A penicillin allergy can be diagnosed by taking a skin test. A tiny amount of the drug will be injected into your skin and that area will then be watched for signs of inflammation. It is important that this is done in the company of a health professional in case of any severe reactions. A RAST (blood test) for penicillin may also be available. However, a negative RAST result does not exclude the possibility of a penicillin allergy.

If you have been diagnosed with a penicillin allergy you can still take antibiotics totally unrelated to penicillin to combat infections. In addition, if you’ve had an allergic reaction to penicillin in the past the allergy may resolve itself over time; many people will lose their allergy to penicillin. It may therefore be useful to be retested for the allergy at a later stage.

If you are in any doubt about taking penicillin you will need to speak to your doctor or specialist who can provide you with the appropriate advice.

Information written by the talkhealth medical panel

Last Reviewed: 25 January 2011
Next Review Date: 25 January 2013