Adverse drug reaction (ADR) - a negative reaction that is caused by the use of drug and warrants either alteration of the drug's dosage or halting administration of the drug.
Adverse effect - same as an adverse drug reaction but not limited to drugs. For example, people who restrict meat in their diet are sometimes at risk for anemia (low blood iron).
Adverse event - a negative outcome observed during the use of a drug (such as during a clinical trial) but not necessarily caused by the drug. For example, in a clinical trial a patient on an experimental diabetes drug may suffer a heart attack - an example of an adverse event. Whether or not the diabetes medication is the cause of the heart attack cannot be determined until all the statistical data have been analyzed at the end of the trial. If the medication is linked to the adverse event, then the adverse event will be re-classified as an ADR. The FDA has a page that lists examples of serious adverse events.
Side effect - a non-clinical term associated with effects that are caused by a drug and yet seemingly unrelated to a drug's intended therapeutic effect. Side effects may be either undesirable or favorable. For example, some women on the combination birth control pill are pleased to observe a reduced incidence of acne.
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