A side effect is basically an unintended occurrence that results from taking a drug. Side effects can be good or bad, depending on how you use the drug.

对于一种药物，副作用基本上是一个意想不到的出现，副作用的好坏取决于你怎么使用这个药物。

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In some cases, drug companies may want to avoid interfering with key proteins to avoid too many unintended side effects, says Marc Vidal, director of the Center for Cancer Systems Biology at Dana-Farber Cancer Institute in Boston.

WSJ: Genes May Link Disparate Diseases