Chances are your doctor has at one time or another considered writing you a prescription for amoxicillin. It's one of the most frequently prescribed medications, and it's commonly used to treat strep throat and ear infections.

But if you have an allergy to penicillin, taking common drugs such as amoxicillin and other medicines in the penicillin family can be dangerous. Allergic responses to penicillin range from annoying rashes to life-threatening reactions, such as difficulty breathing.

Penicillin allergy is the most common drug allergy. Knowing the signs of an allergic reaction can help you spot a serious penicillin allergy before it's too late.

Penicillin belongs to a family of drugs called beta-lactam antibiotics. These drugs include penicillin and amoxicillin, which are relatively inexpensive and generally effective at eradicating many common bacterial infections. These include skin, ear, sinus and upper respiratory infections.

Taken orally or injected, penicillin works by stopping the growth of bacteria in your body. Several different varieties of penicillin exist, and each targets a different infection in a different part of your body. You may have heard of some of the other varieties of penicillin, including:

• Amoxicillin
• Ampicillin
• Dicloxacillin
• Nafcillin
• Penicillin V
• Penicillin G

You aren't born allergic to penicillin, but you can develop an allergy to the drug once you've been exposed to it. After that, re-exposure to penicillin or related antibiotics can trigger an allergic reaction.

Allergic reactions occur because your immune system responds to the drug as if it were a harmful substance instead of a helpful remedy. For reasons not fully understood, your body creates antibodies called immunoglobulin to attack the medication. In most cases of penicillin allergy, the type of immunoglobulin that causes the most problems is type E (IgE).

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How an allergic response occurs

The most common allergic reactions to penicillin are rashes, which aren't life-threatening. Other common reactions include:

• Hives
• Itchy eyes
• Swollen lips, tongue or face (angioedema)

Tell your doctor if you notice any of these or other reactions. He or she can determine whether these are signs of allergic reaction and may prescribe an alternative medication.

In rare instances your allergy can cause an anaphylactic (an-uh-fuh-LAK-tik) reaction, which can be deadly. This type of reaction usually develops within an hour of taking penicillin, and the reaction involves your entire body. In an anaphylactic reaction your airway tubes (bronchi) may constrict, making it hard to breathe, and your blood pressure may drop to life-threatening levels, making you feel dizzy or causing you to lose consciousness. You might also notice:

• Wheezing
• Dizziness
• Loss of consciousness
• Rapid or weak pulse
• Bluing of your skin, including your lips and nail beds
• Diarrhea
• Nausea and vomiting

Seek medical attention immediately if you think you're having an anaphylactic reaction.

Your doctor might recommend an allergy skin test if you need to know whether you're allergic to penicillin — such as when you have an infection that's best treated with penicillin. The test involves injecting a tiny amount of penicillin into your skin and watching that part of your body for signs of inflammation. If the skin test is positive, you're at increased risk of a reaction if you take penicillin or closely related antibiotics.

The best way to avoid an allergic reaction to penicillin is to avoid penicillin and similar antibiotics altogether, if possible. If you're allergic to penicillin, it doesn't mean you're necessarily allergic to all antibiotics. Your doctor may be able to give you an antibiotic that's distantly related to penicillin without causing any problems. Antibiotics completely unrelated to penicillin also are available.

For some infections it may be absolutely necessary for you to take penicillin. If this is the case, your doctor may recommend desensitization. During the desensitization process, you receive small but gradually increasing doses of penicillin orally or intravenously. Because desensitization can trigger an allergic reaction, it's only attempted in a controlled hospital setting — and only when penicillin is absolutely necessary. Your desensitization only lasts as long as you continue taking penicillin. If you stop and then need to take penicillin again later, you'll need to go through the desensitization process again.

Some pregnant women receive penicillin to prevent them from passing group B streptococci (GBS) to their babies — it's generally a safe option for developing babies and it rarely causes fatal infections in newborns. Doctors recommend that all pregnant women be tested for GBS — as many as 30 percent of all women carry the organism. If you're a carrier, your doctor can help you reduce the risk of transmitting the disease to your baby by giving you an antibiotic. Penicillin is the first choice, but if you're allergic, alternative medications are available. Always tell your doctor if you think you're allergic to penicillin.

If you think you've had reactions to penicillin in the past, be sure to tell your doctor or other medical professional. Let your doctor know about any new reactions you notice when taking your medication. If your doctor determines that you're allergic to penicillin, it's a good idea to wear a medical alert bracelet that describes your allergy, or you might want to carry an alert card in your wallet or purse.