Drug firms Pfizer and Flynn Pharma have been fined nearly £90m (€106m) for "excessive and unfair" pricing to the NHS after hiking the cost of an anti-epilepsy drug by up to 2,600% overnight.
The UK's Competition and Markets Authority (CMA) said drug maker Pfizer and distributor Flynn Pharma broke competition law when they increased the cost of a medicine used by around 48,000 patients in the UK.
The watchdog said their moves saw the cost to the NHS of phenytoin sodium capsules rocket from around £2m (€2.3m) a year in 2012 to about £50m (€59m) in 2013 - far more than Pfizer was charging in any other European country.
Pfizer was handed a record £84.2m (€99.7m) fine, while Flynn Pharma was fined £5.2m (€6.1m).
The CMA has also ordered both firms to reduce their prices for the anti-epilepsy drug.
Pfizer rejected the findings and plans to appeal against the decision.
It said its distribution rights deal with Flynn Pharma in September 2012 "represented an opportunity to secure ongoing supply of an important medicine for patients with epilepsy".
"In this transaction, and in all of our business operations, we approached this divestment with integrity, and believe it fully complies with established competition law," it said.
The group added the increased price of the drug was still 25% to 40% lower than the cost of an equivalent medicine by another supplier to the NHS.
Flynn Pharma said it will also appeal against the decision.
David Fakes, chief executive of Flynn Pharma, said: "We believe that left unchallenged, the CMA's decision today would stunt investment in generics, eventually leading to a reduction in supply and less choice for doctors and patients.
"It is a matter of common interest for us to appeal and see this decision overturned."