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

• Hives

• Itching

• Fever

• Swelling

• Shortness of breath

• Wheezing

• Runny nose

• Itchy, watery eyes

• Anaphylaxis

Anaphylaxis

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

• Seizures

• Loss of consciousness

Other conditions 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

• 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.

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.

Penicillins include:

• Amoxicillin

• Ampicillin

• Dicloxacillin

• Nafcillin

• Oxacillin

• Penicillin G

• Penicillin V

• Piperacillin

• Ticarcillin

Cephalosporins include:

• Cefaclor

• Cefadroxil

• Cefazolin

• Cefdinir

• Cefotetan

• Cefprozil

• Cefuroxime

• Cephalexin (Keflex)

• Cefepime (Maxipine)

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