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To most people, “evergreen” refers to a tree that manages to keep its leaves no matter what. But the term has another, highly contentious meaning when discussing prescription drugs — the use of additional patents to extend the monopoly on a medicine and keep cash registers ringing.

Drug makers have long argued their patent modifications reflect substantive enhancements, but the practice has prompted complaints that companies often make minor changes in order to thwart generic competition. And a new study shows the approach may be more pervasive than thought — at least 74 percent of medicines associated with new patents were already on the market.

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