A drug allergy occurs when a person's body develops an immune response against a medication. It can occur if the medication is taken by mouth, injected into the body, or rubbed onto the skin.
Most people with a drug allergy have previously been exposed to that drug or a similar drug. During the earlier exposure, immune cells formed antibodies against the drug. (Antibodies are proteins created by the immune system to battle foreign invaders such as bacteria and viruses.) When a person is exposed to the drug again, the antibodies go into action, setting off the allergic response.
The timing of symptoms can vary. They can occur immediately after starting the medication or days to weeks later.
Drug allergies can pose a significant problem if they prevent the use of a medication that is important for the treatment of a medical condition.
Many people are sensitive to medications, but not all of these sensitivities are true allergic reactions.
Some reactions to medications are well-known side effects. Among the most common side effects that people experience from medications are upset stomach, diarrhea, vomiting, fever and a skin reaction to sunlight called photosensitivity.
These side effects are not the same as drug allergies. Side effects do not involve the immune system, and sometimes can be avoided by lowering the dose of the medication. Drug allergy reactions occur even after using small doses.
The most common drug allergy symptom is a skin rash. If you have been exposed to the drug before, the rash may start quickly, within the first day or two after taking the drug. The reaction may also be delayed and not occur until 8 to 10 days after starting the drug. It is even possible to develop the rash after you have finished a one week course of medication. When this does occur, it is usually related to an antibiotic.
Symptoms that appear within hours of taking the drug are called acute symptoms and usually involve a skin rash, hives or itching. However, in more severe cases, symptoms can advance rapidly to include nasal congestion, rapid pulse rate, drop in blood pressure, difficulty breathing, facial swelling, dizziness and light-headedness. This type of reaction, called anaphylaxis, represents the most serious kind of allergic reaction. Anaphylaxis should be treated with self-injected epinephrine (EpiPen).
A less common type of allergic reaction is called serum sickness. This can occur days or even a week after starting a medication, even if you have never been exposed to the medication before. The symptoms of serum sickness include skin rashes, hives, fever and joint pain. In rare instances, a different type of allergic reaction can cause red blood cells to be destroyed. This is called hemolytic anemia.
If you develop sensitivity to one medication, using other medications with similar chemical structures can be risky. For example, if a person has developed hives or anaphylaxis after taking any of the penicillins, he or she should avoid taking a cephalosporin antibiotic such as cephalexin (Biocef, Keflex, Keftab).
Penicillin and related antibiotics are the most common cause of drug allergies. Other common allergy-causing drugs include:
Some drug reactions are considered idiosyncratic. This means the reaction is an unusual effect of the medication. For example, aspirin can cause nonallergic hives or trigger asthma. Only a small number of these reactions are allergic in nature. Many individuals may confuse an uncomfortable but not serious side effect, such as nausea, with a true drug allergy, which can be life threatening.
Allergic reactions to drugs are usually self-limiting and only last for a few days after the drug is discontinued. In some cases, however, a more severe reaction can occur. Rarely, an allergic skin reaction can cause marked sloughing of the skin, a condition called toxic epidermal necrolysis (TEN). Patients who experience this complication require treatment similar to burn patients. The skin may take months to heal completely. Drugs associated with TEN include sulfa-based antibiotics, allopurinol, and some anticonvulsants.
Treatment depends on the severity of the drug reaction. If you have just a skin rash but otherwise feel well, stopping the medication may be enough. Otherwise, treatment will focus on relieving symptoms. If you suspect a drug reaction, stop taking the drug and contact your health care professional, who can help determine if the reaction is a true drug allergy and suggest an alternative medication if necessary.
Anaphylaxis, the most serious allergic reaction, can cause a dramatic fall in blood pressure, wheezing and breathing difficulties. The most severe cases can lead to loss of consciousness and, rarely, death. Anaphylaxis is treated with an emergency injection of epinephrine (adrenaline) and fluids given intravenously (into a vein).
Some drug allergies can be modified by treatment called drug desensitization. This treatment involves gradually increased doses of the drug over time to create tolerance.
The best way to avoid a medication allergy is to avoid the medication that can cause it, if possible.
Tell your healthcare professional about any history of allergy to food, pollen, certain soaps or cosmetics and other common products. Also disclose any medication reactions you had in the past. Learn about all your medications, whether your doctor prescribed them or you bought them over the counter. Ask your doctor and/or pharmacist which drugs to avoid if you have a history of medication allergy. Whenever dealing with a health care professional who is not familiar with your medical history, tell him or her about your drug sensitivities.
Keep a list in your wallet and consider wearing a medical ID bracelet or necklace.