As U.S. officials press a massive case for alleged generic drug price fixing, authorities in the U.K. have unearthed an example of rivals working a little too closely with each other.

King Pharmaceuticals and Alissa Healthcare Research, which both sold the antidepressant drug nortriptyline, admitted to exchanging "commercially sensitive information" in order to keep prices high.

The U.K.'s Competition and Markets Authority (CMA) said in June it had found the companies exchanged info about prices, volumes and market entry plans for their drugs. Officials opened their investigation back in October 2017, and the allegations covered conduct between 2015 to 2017.

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Even as King and Alissa admitted to the violations, a third company, Lexon Limited, denied them. The investigation against Lexon is ongoing, and the authorities say they're preparing to levy penalties against the company.

RELATED: Lawmakers to Teva, Mylan: Stop stonewalling our price-fixing probe

Meanwhile, a slate of generic drugmakers face U.S. allegations that they worked together to keep prices high. Dozens of states have sued leading generic firms, alleging they met in person and communicated digitally to share info about launch plans and divide up the market. The alleged price-fixing scheme encompassed some of the biggest generics makers in the business, including Teva Pharmaceutical and Mylan, prosecutors say.

In the U.K., competition authorities have taken action against numerous drugmakers in recent years. Pfizer ran into trouble with a 2,600% price hike in the U.K., sparking an investigation and a record £84.2 million fine that was eventually overturned.

RELATED: Pfizer wins appeal of £84M fine in U.K. over 2,600% price hike

Officials in 2016 ordered GlaxoSmithKline to pay £37.6 million over deals with generics companies to delay competition. And in 2017, the CMA targeted similar agreements between Actavis and Concordia to delay competition to hydrocortisone tablets.

Thousands of patients use nortriptyline to relieve symptoms of depression, the Centers for Medicare & Medicaid Services says. The drug’s sales peaked at £38 million in 2015.