Britain's competition watchdog has fined Pfizer a record 84.2 million pounds ($107 million) for its role in ramping up the cost of an epilepsy drug by as much as 2,600 percent.
The Competition and Markets Authority (CMA) also fined Flynn Pharma 5.2 million pounds for overcharging for phenytoin sodium capsules, following a dramatic price hike in 2012.
The CMA's ruling comes amid a growing debate on both sides of the Atlantic about the ethics of price hikes for old off-patent medicines that are only made by a few firms and where there is little competition.
US drugmaker Turing Pharmaceuticals, led at the time by hedge fund manager Martin Shkreli, caused outrage last year by raising the US price of Daraprim, an old anti-infective drug, by more than 5,000 percent to $750 a pill.
In the case of phenytoin sodium capsules, the UK price charged for 100 mg packs of the drug jumped from 2.83 pounds to 67.50 in 2012, before reducing to 54.00 from May 2014.
As a result, annual spending on the capsules by Britain's National Health Service rose from 2 million pounds in 2012 to about 50 million in 2013. The CMA said UK prices were many times higher than elsewhere in Europe.
Pfizer used to market the medicine under the brand name Epanutin but sold the rights to Flynn, a privately owned British company, in September 2012.
It was then debranded, meaning that it was no longer subject to price regulation, and the price soared.
"The companies deliberately exploited the opportunity offered by debranding to hike up the price for a drug which is relied upon by many thousands of patients," Philip Marsden, chairman of the CMA's case decision group, said on Wednesday.
“This is the highest fine the CMA has imposed and it sends out a clear message to the sector that we are determined to crack down on such behaviour."
Pfizer said in a statement it planned to appeal all aspects of the CMA's verdict.