Medically reviewed by Drugs.com. Last updated on Sep 29, 2021.
Penicillin allergy is an abnormal reaction of your immune system to the antibiotic drug penicillin. Penicillin is prescribed for treating various bacterial infections.
Common signs and symptoms of penicillin allergy include hives, rash and itching. Severe reactions include anaphylaxis, a life-threatening condition that affects multiple body systems.
Research has shown that penicillin allergies may be over-reported — a problem that can result in the use of less-appropriate and more-expensive antibiotic treatments. Therefore, an accurate diagnosis is needed when penicillin allergy is suspected to ensure the best treatment options in the future.
Other antibiotics, particularly those with chemical properties similar to penicillin, also can result in allergic reactions.
Signs and symptoms of penicillin allergy often occur within an hour after taking the drug. Less commonly, reactions can occur hours, days or weeks later.
Penicillin allergy signs and symptoms may include:
- Skin rash
- Shortness of breath
- Runny nose
- Itchy, watery eyes
Anaphylaxis is a rare, life-threatening allergic reaction that causes the widespread dysfunction of body systems. Signs and symptoms of anaphylaxis include:
- Tightening of the airways and throat, causing trouble breathing
- Nausea or abdominal cramps
- Vomiting or diarrhea
- Dizziness or lightheadedness
- Weak, rapid pulse
- Drop in blood pressure
- Loss of consciousness
Delayed reactions resulting from penicillin allergy
Less-common penicillin allergy reactions occur days or weeks after exposure to the drug and may persist for some time after you stop taking it. These conditions include:
- Serum sickness, which may cause fever, joint pain, rash, swelling and nausea
- Drug-induced 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 results in rash, high white blood cell counts, general swelling, swollen lymph nodes and recurrence of dormant hepatitis infection
- Stevens-Johnson syndrome or toxic epidermal necrolysis, which involves severe blistering and peeling of the skin
- Inflammation in 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 of penicillin — as happens with other medications — 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 for which you are being treated — or unrelated symptoms — also may be mistaken as an allergic drug reaction.
When to see a doctor
See your doctor as soon as possible if you experience signs or symptoms of penicillin allergy. It's important to understand and discuss what is an allergic reaction, what is a typical side effect and what you can tolerate in taking a medication.
Call 911 or emergency medical help if you experience signs of a severe reaction or suspected anaphylaxis after taking penicillin.
A drug rash usually starts within two weeks of taking a new medication and begins as discrete red spots that spread, covering large areas of the body.
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.
- Penicillin G
- Penicillin V
- Cefepime (Maxipine)
- Cephalexin (Keflex)
While 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
- A family history of drug allergy
- Increased exposure to penicillin, because of high doses, repetitive use or prolonged use
- Certain illnesses commonly associated with allergic drug reactions, such as infection with HIV or the Epstein-Barr virus
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.
A thorough exam and appropriate diagnostic tests are essential for an accurate diagnosis. A misdiagnosed penicillin allergy may result in the use of less-appropriate or more-expensive antibiotics.
Your doctor will conduct a physical examination, ask questions about your symptoms and order additional tests. You may be referred to an allergy specialist (allergist) for these tests. These may include the following.
With a skin test, the allergist or nurse administers a small amount of the suspect penicillin to your skin with a tiny needle. A positive reaction to a test will cause a red, itchy, raised bump.
A positive result indicates a high likelihood of penicillin allergy. A negative test result usually means you're not at high risk of an allergy to penicillin. But a negative result is more difficult to interpret because some kinds of drug reactions cannot be detected by skin tests.
If the diagnosis of a penicillin allergy is uncertain, a graded drug challenge may be recommended. With this procedure, you receive up to five doses of the suspect penicillin, starting with a small dose and increasing to the desired dose. If you reach the therapeutic dose with no reaction, then your doctor will conclude you aren't allergic to that type of penicillin. You will be able to take the drug as prescribed.
If you are allergic to one type of penicillin, your doctor may recommend a graded challenge with a type of penicillin or cephalosporin that's less likely — because of known chemical properties — to cause an allergic reaction. This would enable your doctor to identify an antibiotic that can be used safely.
During a drug challenge, your doctor provides careful supervision, and supportive care services are available to treat an adverse reaction.
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.
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 an 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.