Penicillin Allergy – Symptoms and Causes


Penicillin allergy is an abnormal reaction of your immune system to the antibiotic penicillin. Penicillin is prescribed to treat various bacterial infections.

Common signs and symptoms of penicillin allergy include hives, rashes, and itching. Serious reactions include anaphylaxis, a life-threatening condition that affects multiple bodily systems.

Research has shown that penicillin allergies can be over-reported – a problem that can lead to the use of less appropriate and more expensive antibiotic treatments. Therefore, an accurate diagnosis is necessary when a penicillin allergy is suspected to ensure the best treatment options in the future.

Other antibiotics, especially those with similar chemical properties to penicillin, can also cause allergic reactions.



Signs and symptoms of penicillin allergy often appear within an hour of taking the drug. Less commonly, reactions can occur hours, days or weeks later.

Signs and symptoms of penicillin allergy may include:

  • itchy skin
  • Urticaria
  • Itching
  • Fever
  • Swelling
  • Shortness of breath
  • Wheezing
  • Runny nose
  • Itchy, watery eyes
  • Anaphylaxis


Anaphylaxis is a rare and life-threatening allergic reaction that causes generalized dysfunction of body systems. Signs and symptoms of anaphylaxis include:

  • Tightening of the airways and throat, causing breathing difficulties
  • Nausea or abdominal cramps
  • Vomiting or diarrhea
  • Dizziness or dizziness
  • Weak and rapid pulse
  • Drop in blood pressure
  • Seizures
  • Loss of consciousness

Delayed reactions resulting from penicillin allergy

Less common penicillin allergic reactions occur days or weeks after exposure to the drug and may persist for some time after treatment is stopped. These conditions include:

  • serum sickness, which can cause fever, joint pain, rash, swelling, and nausea
  • drug anemia, a reduction in red blood cells, which can cause fatigue, irregular heartbeats, shortness of breath, and other signs and symptoms
  • Drug reaction with eosinophilia and systemic symptoms (DRESS), which leads to rash, high white blood cell count, general swelling, swollen lymph nodes, and recurrence of dormant hepatitis
  • Stevens-Johnson syndrome or toxic epidermal necrolysis, which involves severe blistering and peeling of the skin
  • Inflammation of the kidneys (nephritis), which can cause fever, blood in the urine, general swelling, confusion, and other signs and symptoms

Adverse events that are not allergic reactions

You may experience side effects from penicillin – as happens with other drugs – that are not an allergic reaction to the drug. Depending on the type of penicillin, common side effects may include mild nausea or diarrhea, headache, or vaginal itching. Signs or symptoms of an infection you are being treated for – or unrelated symptoms – can also be mistaken for an allergic reaction to a drug.

When to consult a doctor

See your doctor as soon as possible if you have any signs or symptoms of penicillin allergy. It is important to understand and discuss what an allergic reaction is, a typical side effect, and what you can tolerate while taking a medication.

Call 911 or emergency medical help if you have signs of a severe reaction or suspect anaphylaxis after taking penicillin.

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Penicillin allergy occurs when your immune system becomes oversensitive 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 becomes sensitive to penicillin, you must be exposed to the drug at least once. If and when your immune system mistakenly identifies penicillin as a harmful substance, it develops an antibody against the drug.

The next time you take the drug, these specific antibodies will signal it and direct the immune system’s attacks on the substance. The 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 traces of it in the food supply may be enough for a person’s immune system to create an antibody against it.

Penicillins and related drugs

Penicillins belong to a class of antibacterial drugs called beta-lactam antibiotics. Although the mechanisms of the drugs vary, they generally 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 have had an allergic reaction to one type of penicillin, you may – but need not – be allergic to other types of penicillin or to certain 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)

Risk factors

Although anyone can have an allergic reaction to penicillin, a few factors can increase your risk. These include:

  • A history of other allergies, such as food allergy or hay fever
  • Allergic reaction to another drug
  • Family history of drug allergy
  • Increased exposure to penicillin, due to high doses, repetitive use, or prolonged use
  • Certain diseases commonly associated with allergic reactions to drugs, such as infection with HIV or the Epstein-Barr virus


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

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

September 29, 2021

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