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Most people have probably experienced an unwanted side effect to a medicine at some time in their lives. Many drugs commonly cause side effects, such as an upset stomach after taking aspirin or drowsiness after taking a cold medication. Adverse drug reactions also can be quite serious; they account for an estimated 106,000 deaths each year in the United States. As more medications are approved each year, the problem is expected to grow.
An adverse drug reaction is any effect not intended by proper administration of a medication. Reactions also can occur between medications, even nonprescription ones. Most adverse drug reactions - more than 90 percent - do not involve the immune system. When the immune system is involved, a person is said to have drug hypersensitivity. Allergy is one type of hypersensitivity reaction.
What is drug hypersensitivity?
Medications can cause unwanted reactions in many ways. Sometimes, it's a direct effect of the drug on the body. Drug hypersensitivity reactions occur when the immune system responds to a medication or to the biologic products that result when the body breaks down a medication. In some cases, the immune system tries to attack the substance, causing symptoms of the drug reaction. Drugs also can cause allergic reactions similar to those caused by bee stings or other allergenic substances.
People who have a family history of allergic diseases may be more likely to have drug allergy, but are not at greater risk to develop non-allergic types of reactions. Fortunately, a family history of allergy to a particular drug does not increase a person's chance of being allergic to that same drug.
A person must have a previous exposure to a drug in order to have a true allergic reaction to it. Such reactions most often occur when a drug is administered intravenously or by injection, delivery methods that send the drug directly to the bloodstream. Reactions occur less frequently when drugs are taken by mouth. The chance of an allergic reaction increases when a medication is administered frequently or in large doses.
Certain medications are more likely to cause allergic reactions than others due to their chemical structure. Penicillin and other antibiotics are some of the most common culprits of allergic drug reactions. Penicillin, however, can also cause other types of immune reactions, as well as reactions that do not involve the immune system.
Symptoms
The most common types of allergic reactions to a drug are:
Skin rash or hives
Itchy skin
Wheezing or other breathing problems
Swelling of body parts
Anaphylaxis, a life-threatening allergic reaction
While these are the most common symptoms of drug allergy, adverse reactions can occur in any organ or system of the body.
Allergic reactions can occur within minutes or hours of exposure to a medication. Drug reactions can even occur some time after a medication has been stopped. For example, a person may develop a rash or hives a week after stopping a medication.
A "pseudoallergic," or anaphylactoid, reaction does not involve allergic antibodies and can occur without prior exposure. Symptoms are similar to a true allergic reaction: a person may develop a rash or hives, have difficulty breathing, and experience swelling of body parts. Common causes of pseudoallergic reactions include aspirin and X-ray dye.
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