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 exclusively.

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 (Aging and Drugs), the brain is commonly affected, often resulting in drowsiness and confusion.

Some adverse drug reactions are identified when a drug is being tested before it is submitted to the Food and Drug Administration (FDA) for approval. Other adverse drug reactions, typically those that are uncommon, are not detected until the drug has been on the market long enough to be used by a large number of people. Thus, doctors are required to report suspected adverse drug reactions to the FDA.

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Some Serious Adverse Drug Reactions Adverse Drug Reaction Types of Drugs Examples Anemia (resulting from decreased production or increased destruction of red blood cells) Certain antibiotics Chloramphenicol Drugs used to treat malaria or tuberculosis in people with G6PD enzyme deficiency Chloroquine Isoniazid Primaquine Angioedema (swelling of the lips, tongue, and throat causing difficulty breathing) ACE inhibitors Captopril Enalapril Lisinopril Bone fractures Proton pump inhibitors Esomeprazole Lansoprazole Omeprazole Blood clots Birth control drugs (all forms including patches and pills) Drospirenone/ethinyl estradiol Norelgestromin/ethinyl estradiol Confusion and drowsiness Sedatives, including many antihistamines Diphenhydramine Antidepressants (especially in older people) Amitriptyline Imipramine Decreased production of white blood cells, with increased risk of infection Certain antipsychotic drugs Clozapine Chemotherapy drugs Cyclophosphamide Mercaptopurine Methotrexate Vinblastine Some drugs used to treat thyroid disorders Propylthiouracil Kidney damage NSAIDs (repeated use of excessive doses) Ibuprofen Ketoprofen Naproxen Aminoglycoside antibiotics Gentamicin Kanamycin Some chemotherapy drugs Cisplatin Methotrexate Antifungals Amphotericin B Some antibiotics Gentamicin Tetracycline (outdated) Liver damage Some analgesics Acetaminophen (use of excessive doses) Some drugs used to treat tuberculosis Isoniazid Iron supplements (in excessive amounts) — Antidepressants Duloxetine Antibiotics Tetracycline Muscle tissue destruction (rhabdomyolysis) Statins Atorvastatin Simvastatin Stomach or intestinal ulcers (with or without bleeding) NSAIDs Aspirin Ibuprofen Ketoprofen Naproxen Anticoagulants Heparin Warfarin Bisphosphonates Alendronate Etidronate Risedronate Toxic epidermal necrolysis (see Itching and Noninfectious Rashes: Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis) Some antibiotics Penicillins Quinolones Anticonvulsants Phenytoin Valproic acid Ventricular tachycardia (see Abnormal Heart Rhythms: Ventricular Tachycardia) Antiarrhythmics Amiodorone Procainamide Sotalol Antipsychotics Chlorpromazine Haloperidol Lithium ACE = angiotensin-converting enzyme; G6PD = glucose-6-phosphate dehydrogenase; NSAIDs = nonsteroidal anti-inflammatory drugs. Some Serious Adverse Drug Reactions Adverse Drug Reaction Types of Drugs Examples Anemia (resulting from decreased production or increased destruction of red blood cells) Certain antibiotics Chloramphenicol Drugs used to treat malaria or tuberculosis in people with G6PD enzyme deficiency Chloroquine Isoniazid Primaquine Angioedema (swelling of the lips, tongue, and throat causing difficulty breathing) ACE inhibitors Captopril Enalapril Lisinopril Bone fractures Proton pump inhibitors Esomeprazole Lansoprazole Omeprazole Blood clots Birth control drugs (all forms including patches and pills) Drospirenone/ethinyl estradiol Norelgestromin/ethinyl estradiol Confusion and drowsiness Sedatives, including many antihistamines Diphenhydramine Antidepressants (especially in older people) Amitriptyline Imipramine Decreased production of white blood cells, with increased risk of infection Certain antipsychotic drugs Clozapine Chemotherapy drugs Cyclophosphamide Mercaptopurine Methotrexate Vinblastine Some drugs used to treat thyroid disorders Propylthiouracil Kidney damage NSAIDs (repeated use of excessive doses) Ibuprofen Ketoprofen Naproxen Aminoglycoside antibiotics Gentamicin Kanamycin Some chemotherapy drugs Cisplatin Methotrexate Antifungals Amphotericin B Some antibiotics Gentamicin Tetracycline (outdated) Liver damage Some analgesics Acetaminophen (use of excessive doses) Some drugs used to treat tuberculosis Isoniazid Iron supplements (in excessive amounts) — Antidepressants Duloxetine Antibiotics Tetracycline Muscle tissue destruction (rhabdomyolysis) Statins Atorvastatin Simvastatin Stomach or intestinal ulcers (with or without bleeding) NSAIDs Aspirin Ibuprofen Ketoprofen Naproxen Anticoagulants Heparin Warfarin Bisphosphonates Alendronate Etidronate Risedronate Toxic epidermal necrolysis (see Itching and Noninfectious Rashes: Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis) Some antibiotics Penicillins Quinolones Anticonvulsants Phenytoin Valproic acid Ventricular tachycardia (see Abnormal Heart Rhythms: Ventricular Tachycardia) Antiarrhythmics Amiodorone Procainamide Sotalol Antipsychotics Chlorpromazine Haloperidol Lithium ACE = angiotensin-converting enzyme; G6PD = glucose-6-phosphate dehydrogenase; NSAIDs = nonsteroidal anti-inflammatory drugs.

Last full review/revision October 2012 by Joan B. Tarloff, PhD