Drug toxicity is an unpleasant and potentially dangerous effect of a drug. Idiosyncrasy is

an example of an unpredictable type of drug toxicity. This is any unexpected and uncommon

side effect that develops after administration of a drug. For example, in some people,

penicillin causes an idiosyncratic reaction, such as anaphylaxis (acute hypersensitivity

with asthma and shock). Anaphylaxis occurs as a result of exposure to a previously

encountered drug or foreign substance (antigen).

Other types of drug toxicity are more predictable and expected, such as stomach upset

after aspirin use. Physicians are trained to be aware of the potential toxic effects of all drugs

that they prescribe. Iatrogenic (produced by treatment) disorders can occur, however, as

a result of mistakes in drug use or because of individual sensitivity to a given agent.

Side effects are unpleasant effects that routinely result from the use of a drug. They

often occur with the usual therapeutic dosage of a drug and generally are tolerable and

reversible when the drug is discontinued. For example, nausea, vomiting, and alopecia are

common side effects of the chemotherapeutic drugs used to treat cancer. Other, rare side

effects may be life-threatening, such as severe allergic reactions.

Contraindications are factors in a patient's condition that make the use of a particular

drug dangerous and ill-advised. For example, in the presence of kidney failure, it is unwise

to administer a drug, such as methotrexate, that is normally eliminated by the kidneys

because excess drug will accumulate in the body and cause adverse effects.

Drug resistance is the reduction in effectiveness of a drug. It is seen when drugs are

unable to control the disease process in a particular patient. Resistance results from an

important mutation (genetic change) in the disease (bacterial, viral, or cancer.)