Penicillin Allergy

Penicillin is the most common drug allergy, affecting up to 10% of the population. Most patients lose their sensitivity over time, but there are some that continue to live with the allergy. Penicillin allergies are commonly over-reported and result in an unnecessary use of stronger antibiotics. This is why it’s important to test for this allergy after a certain amount time to confirm or disprove a previous label of penicillin allergy.

Although about 80% of patients lose their sensitivity after 10 years, penicillin is still one of the most common causes of anaphylaxis. Drug eruptions, an adverse reaction of the skin, have also been reported. In this case, cutaneous reactions appear as mild symptoms and disappear when the drug is withdrawn. More serious drug eruptions include organ injury.

Penicillin Allergy Symptoms

If you experience any of these symptoms after taking penicillin-containing medication, contact your doctor immediately. Symptoms usually emerge instantly; however, it’s not uncommon for symptoms to appear hours later. It is also common for allergic reactions to occur after the second exposure. The first time you take penicillin, your body might not register it as a harmful substance.

Common Penicillin Medications


These medications are administered by shot or orally. They are all effective in treating bacterial infections. Amoxicillin is more popular than other penicillin-containing medications. It is inexpensive and widely available. If a doctor suggests amoxicillin for treatment, make sure you complete an allergy panel first.

There are also some differences among the penicillin-containing medications. Ampicillin and oxacillin are semi-synthetic penicillins and are taken orally. They are used to fight bacteria that are more resistant to antibiotics.

Piperacillin is administered by shot and used in cases where the bacteria is hard to kill, such as urinary tract infections. It’s usually given with amoxicillin. Dicloxacillin and nafcillin are naturally resistant to some bacteria.  If you are allergic to one penicillin-containing medication, you cannot take any of the other medications. You should ask about a penicillin alternative to treat your infection.

Penicillin Alternatives


Trimethoprim and nitrofurantoin can be alternatives for piperacillin, treating urinary tract infections. Doxycycline, erythromycin, and clarithromycin treat upper respiratory infections; and cotrimoxazole is available as an alternative to treat abdominal infections. Your doctor will explain which penicillin alternative fits your situation.

Diagnosing A Penicillin Allergy

Most allergists will use a skin prick test to diagnose a penicillin allergy; however, many patients may still receive a negative result even if it is believed they are allergic. In this case, an allergist will introduce the allergen beneath the skin in a method called intradermal testing.

In addition, a penicillin challenge may be performed. A dose of penicillin-containing medication (usually amoxicillin) may be administered in the office under careful observation to confirm a negative dose. This is done only as a last option, as penicillin is known to cause severe allergic reactions. Your doctor will have epinephrine and other allergy medications on hand to treat any symptoms.

Penicillin Allergy Treatment

Avoidance is the best treatment for any allergy. There are many alternative medications available to treat your bacterial infections. If you avoid penicillin, there is a chance that your sensitivity will disappear or weaken. In the meantime, visit your allergist for recommended allergy medications to treat minor symptoms.

For more serious allergies, desensitization is another treatment option. Your doctor will gradually administer the medication by mouth or as shots to build tolerance of the drug. Desensitization is recommended for patients that require treatment with penicillin, such as those suffering from neurosyphilis. This chronic condition can only be treated with penicillin-containing medication. Ask your allergist if desensitization is necessary for you.

Frequently Asked Questions

Does Penicillin Come from Mold?

Penicillium mold naturally produces the antibiotic penicillin. However, scientists separated the penicillin product from the mold and purified it for use as an antibiotic.

What Other Antibiotics are Safe for People That are Allergic to Penicillin?

Bactrim, cephalosporins, and macrolides are other alternatives to penicillin for treating bacterial infections. Always consult with your doctor about taking new medications.

Are There Non-allergy Related Side Effects to Penicillin?

Side effects of penicillin include nausea, vomiting, abdominal pain, diarrhea, and upset stomach.

Who Is Likely to Develop a Penicillin Allergy?

Patients who are constantly exposed to the drug are most likely to develop a penicillin allergy, this concerns patients with serious infections.


Only about 10% of people have a true penicillin allergy, but symptoms can be life threatening. Contact the allergists at NY Allergy & Sinus Centers if you believe you have an allergy or sensitivity to penicillin. Doctors recommend getting allergy tested early to confirm your allergy, and then again after a few years to see if the allergy still exists. Call (212) 686-4448 to book your appointment today!