In 2016, a blockbuster drug called Humira was poised to become a lot less valuable. The key patent on the best-selling anti-inflammatory medication, used to treat conditions like arthritis, was expiring at the end of the year. Regulators had blessed a rival version of the drug, and more copycats were close behind. The onset of competition seemed likely to push down the medication’s $50,000-a-year list price.

Instead, the opposite happened. Through its savvy but legal exploitation of the U.S. patent system, Humira’s manufacturer, AbbVie, blocked competitors from entering the market.

In February, the curtain is expected to come down on a monopoly that has generated $114 billion in revenue for AbbVie just since the end of 2016. The knockoff drug that regulators authorized more than six years ago, Amgen’s Amjevita, will come to market in the United States, and as many as nine more Humira competitors will follow this year from pharmaceutical giants including Pfizer.

AbbVie orchestrated the delay by building a formidable wall of intellectual property protection and suing would-be competitors before settling with them to delay their product launches until this year. The strategy has been a gold mine for AbbVie, at the expense of patients and taxpayers.

According to the New York Times, federal courts have upheld the legality of AbbVie’s patent strategy with Humira, though lawmakers and regulators over the years have proposed changes to the U.S. patent system to discourage such tactics.

Patents are good for 20 years after an application is filed. Because they protect patent holders’ right to profit off their inventions, they are supposed to incentivize the expensive risk-taking that sometimes yields breakthrough innovations. But drug companies have turned patents into weapons to thwart competition.

AbbVie and its affiliates have applied for 311 patents, of which 165 have been granted, related to Humira, according to the Initiative for Medicines, Access and Knowledge, which tracks drug patents. A vast majority were filed after Humira was on the market.

Some of Humira’s patents covered innovations that benefited patients, like a formulation of the drug that reduced the pain from injections. But many of them simply elaborated on previous patents.

Last year, the company’s tactics became a rallying cry for federal lawmakers as they successfully pushed for Medicare to have greater control over the price of widely used drugs that, like Humira, have been on the market for many years but still lack competition.