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A side effect is an effect of a drug other than that which is intended or desired. Usually, a side effect refers to an undesirable effect such as nausea, headache, insomnia, or other symptom, which occurs when an individual takes a drug intended to relieve some other condition. Side effects can range in severity, and in some cases side effects may be severe enough that drug treatment should be stopped or dosage reduced. Side effects can also occur when two or more medications, each safe enough in isolation, interact to produce undesirable consequences (see: drug interaction).