King Pharmaceuticals and Alissa Healthcare Research , suppliers of antidepressant drug Nortriptyline, have admitted to illegally sharing information to maintain high prices across the UK market.

The move comes after the UK Competition and Markets Authority (CMA)’s probe, which also includes a third firm, Lexon .

Prescribed by the country’s National Health Service (NHS), Nortriptyline provides relief from symptoms in depression sufferers. The drug cost the NHS £38m in 2015 when its spending peaked.

In June, the CMA accused that King, Alissa and Lexon illegally exchanged information on prices, volumes and entry plans in a bid to maintain high prices for Nortriptyline.

According to the competition watchdog, Auden Mckenzie agreed in 2014 to supply only 10mg tablets, while King would supply only 25mg tablets of the drug.

Accusations state that the two companies also fixed the supply quantities and prices.

In a Statement of Objections, CMA director of antitrust Geoff Steadman said: “If pharmaceutical companies get together to restrict competition for the supply of a drug, this can lead to the NHS and ultimately the UK taxpayer, paying over the odds for what are often essential medical treatments.

“We expect drug suppliers to abide by competition law so that the NHS is not denied the opportunity of benefitting from lower prices for medicines.”

In the latest development, King and Alissa have admitted to sharing commercially sensitive information with each another and with Lexon from 2015-2017.

Lexon has denied any participation in the alleged activities; the CMA is now investigating the firm.

Alissa and King are expected to face a fine for breaking competition law after the CMA completes its investigation.

In May, CMA alleged that Lexon, along with Alliance Pharmaceuticals, Focus and Medreich engaged in anti-competitive behaviour for anti-nausea drug, prochlorperazine.