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In the early 1900s, German scientist Paul Ehrlich described an ideal drug as a "magic bullet." Such a drug would be aimed precisely at a disease site and would not harm healthy tissues. Although many new drugs are aimed more accurately than their predecessors, none of them, as of yet, hit the target precisely.

Most drugs produce several effects, but usually only one effect—the therapeutic effect—is wanted for the treatment of a disorder. The other effects may be regarded as unwanted, whether they are intrinsically harmful or not. For example, certain antihistamines cause drowsiness as well as control the symptoms of allergies. When an over-the-counter sleep aid containing an antihistamine is taken, drowsiness is considered a therapeutic effect. But when an antihistamine is taken to control allergy symptoms during the daytime, drowsiness is considered an annoying, unwanted effect.

Most people, including health care practitioners, refer to unwanted effects as side effects; another term used is adverse drug event. However, the term adverse drug reaction is technically more appropriate for drug effects that are unwanted, unpleasant, noxious, or potentially harmful.

Not surprisingly, adverse drug reactions are common. Most adverse drug reactions are relatively mild, and many disappear when the drug is stopped or the dose is changed. Some gradually subside as the body adjusts to the drug. Other adverse drug reactions are more serious and last longer. About 3 to 7% of all hospital admissions in the United States are for treatment of adverse drug reactions. Adverse drug reactions occur during 10 to 20% of hospital admissions, and about 10 to 20% of these reactions are severe.

Digestive disturbances—loss of appetite, nausea, a bloating sensation, constipation, and diarrhea—are particularly common adverse drug reactions, because most drugs are taken by mouth and pass through the digestive tract. However, almost any organ system can be affected. In older people, the brain is commonly affected, often resulting in drowsiness and confusion.

Some Serious Adverse Drug Reactions Adverse Drug Reaction Types of Drugs Examples Peptic ulcers or bleeding from the stomach Corticosteroids taken by mouth or by injection (not those applied to the skin in creams or lotions) HydrocortisoneSome Trade Names See cortisol Prednisone Nonsteroidal anti-inflammatory drugs AspirinSome Trade Names BAYER IbuprofenSome Trade Names ADVIL MOTRIN Ketoprofen NaproxenSome Trade Names ALEVE ANAPROX NAPROSYN Anticoagulants Heparin WarfarinSome Trade Names COUMADIN Anemia (resulting from decreased production or increased destruction of red blood cells) Certain antibiotics ChloramphenicolSome Trade Names CHLORAMPHENICOL Some nonsteroidal anti-inflammatory drugs Phenylbutazone (not available in the United States) Drugs used to treat malaria or tuberculosis in people with G6PD enzyme deficiency ChloroquineSome Trade Names ARALEN Isoniazid Primaquine Decreased production of white blood cells, with increased risk of infection Certain antipsychotic drugs ClozapineSome Trade Names CLOZARIL Chemotherapy drugs CyclophosphamideSome Trade Names LYOPHILIZED CYTOXAN MercaptopurineSome Trade Names PURINETHOL MethotrexateSome Trade Names TREXALL Vinblastine Some drugs used to treat thyroid disorders Propylthiouracil Liver damage Some analgesics AcetaminophenSome Trade Names TYLENOL (use of excessive doses) Some drugs used to treat tuberculosis Iron supplements (in excessive amounts) Isoniazid Kidney damage Nonsteroidal anti-inflammatory drugs (repeated use of excessive doses) IbuprofenSome Trade Names ADVIL MOTRIN Ketoprofen NaproxenSome Trade Names ALEVE ANAPROX NAPROSYN Aminoglycoside antibiotics Gentamicin Kanamycin Some chemotherapy drugs CisplatinSome Trade Names PLATINOL Confusion and drowsiness Sedatives, including many antihistamines DiphenhydramineSome Trade Names BENADRYL Antidepressants (especially in older people) Amitriptyline ImipramineSome Trade Names TOFRANIL

Last full review/revision August 2007 by Joan B. Tarloff, PhD