US drugs giant Pfizer has challenged the record £84 million fine imposed by the UK’s Competition and Markets Authority (CMA) for a supply scheme that resulted in a 2,600% price overnight increase on an epilepsy drug sold to the country’s National Health System.

Flynn Pharma, the drug’s US-based distributor, has separately filed an appeal challenging its £5.2 million fine. The two companies said the decision should be annulled and the fines set aside or reduced.

According to CMA, the UK health authority’s spending on the phenytoin sodium capsules used by 48,000 patients in the UK rose from £2 million in 2012 to around £50 million in 2013 and were “many times” higher than Pfizer charged in other European countries.

Both Pfizer and Flynn disputed the CMA findings that they were “dominant” players in the market for the drug, claiming also that the authority “ignored market realities” in saying the prices were exorbitant. In fact, they said, said the authority did not take into account that the price charged by the distributor was less than what the NHS paid for a comparable version of the drug from another supplier.

In the meantime Pfizern said it has reduced the price at which it supplies phenytoin sodium capsules to Flynn Pharma to the price suggested by CMA.  In the past, the US drugmaker defended its arrangement with Flynn as a chance to secure ongoing supply of an important medicine that it had been having to sell at a loss.