But drugmakers often extend their patents by making small tweaks to the drugs, sustaining their monopolies for several years.

Legal experts refer to this tactic as “evergreening,” said Stanford University law professor Mark Lemley, who was not involved with the I-MAK report.

The practice of extending patents doesn’t always go unchallenged. In some cases, generic manufacturers sue the drugmakers to get their drugs on the market, Lemley said. However, he added, those lawsuits often end in settlements between the companies.

Settling the lawsuit is “essentially paying the generic to go away,” Lemley said.