A drug allergy, not to be confused with side effects or an overdose, occurs when the immune system mistakes a drug for a harmful substance and releases chemicals to defend against it. These chemicals cause the symptoms. Drug allergies can be fatal, and are responsible for more deaths than any other type of allergy.
Any drug, including some sold over-the-counter, and even natural herbal remedies such as echinacea, can cause an allergic reaction. However, the most common cause of drug allergy is penicillin, which according to some estimates affects as many as 10% of the population. Other drugs that are sometimes associated with allergies include antibiotics (particularly ones known as sulfa drugs), aspirin, ibuprofen, various drugs used for the treatment of HIV and AIDS, some chemotherapy drugs, insulin and some anesthetics, including local anesthetics. An allergy to one type of drug does not necessarily mean that you will be allergic to others, although it increases the likelihood that you will be.
For those who have had an allergic reaction to medication, it is essential to work out what drug caused the reaction, and which related drugs might also be harmful. The individual must take personal responsibility for alerting medical and dental staff to their condition in the future.
Symptoms can appear within minutes or hours after administration of the drug. A reaction is possible even if the drug has been taken safely before, as it is possible for the body to have built up resistance in the meantime.
The most common symptoms include hives (swollen red welts on the skin), itching, a rash or fever. Slightly less common symptoms include difficulty breathing, dizziness, swelling of the face, mouth or throat or vomiting. There is also a small risk of life-threatening anaphylaxis, a severe, often sudden reaction where the body goes into shock. Symptoms of anaphylaxis include difficulty breathing, flushing, racing or weak pulse, dizziness, a sense of dread and loss of consciousness. Anaphylaxis is a medical emergency and must be treated with immediate administration of epinephrine (adrenaline).
An allergic reaction can occur whether the drug was taken orally or by injection. The more severe reactions are more likely in situations where the drugs have been injected. Luckily this is much more likely to occur in situations, such as during surgery, where emergency treatment is readily available.
Some people experience an upset stomach after taking medication. If this is the only symptom it is unlikely to be an allergic reaction and more likely to result from sensitivity to the substance.
A drug allergy can be quite difficult to diagnose. Some of the symptoms, such as a rash or dizziness, may be caused by the infection which the drug is being used to treat. Some symptoms may simply be side effects of the drug (listed on the packet) rather than an allergic reaction.
If you believe that you may be allergic to a drug or have had any adverse reaction after taking a drug you should talk to your doctor. You are likely to be asked what you suspect caused your reaction, what drugs or herbal remedies you have been taking and what reactions you experienced.
For some drug allergies, such as penicillin, your doctor may decide to do a skin-prick test. This involves putting a tiny amount of the substance on the skin and piercing it with a needle (it does not hurt) to test for a reaction. A red lump would indicate an allergic response. However, where you have had a severe reaction in the past, or where alternative drugs are available, your doctor may decide that the benefits of a skin-prick test are outweighed by the risks.
In some cases a blood test may be carried out. However these are of limited accuracy in diagnosing drug allergies and are more useful in working out whether symptoms could have been caused by some other condition.
As a last resort, a drug challenge may be done. During this procedure, which must only be done under close medical supervision, you will be given tiny doses of the drug to see if this causes a reaction. This is not an option where you have experienced a severe reaction to the drug.
After carrying out whichever, if any, tests are appropriate, the allergist may be able to confirm that you are allergic to a given drug. However there is a possibility that tests will be inconclusive. In either case an appropriate course of action needs to be discussed with your doctor.
Management of the condition
If you have been diagnosed as being allergic to a specific type of drug then it is up to you to always inform medical and even dental staff of this diagnosis. It may be a good idea to wear a tag or bracelet which signals your allergy.
Sometimes, all that is needed is for you to stop using the drug or medicine. In cases of mild symptoms such as hives, antihistamines can be used. Where the reaction is more severe, corticosteroids may be given. If anaphylaxis occurs, this must be treated with immediate injection of epinephrine and hospital admittance.
Your doctor will avoid prescribing a drug which you are allergic to. If there is no alternative treatment, providing the drug or medicine has not caused you to experience life-threatening symptoms in the past, drug desensitization may be recommended. This is done under close medical supervision and involves giving the drug in very small quantities at first and then building up the intake over hours or even days. The theory is that this will build up your tolerance to the drug. You will probably need to repeat drug desensitization if you need to start taking the drug again after having stopped taking it every day, for example , where you are being given a second dose of chemotherapy.