LONDON  - Drug company Concordia overcharged Britain's health service millions of pounds for an essential thyroid drug by abusing its position as the only supplier, the country's Competition and Markets Authority said on Tuesday.

The National Health Service spent 34 million pounds (US$45 million) on liothyronine tablets last year, up from 600,000 pounds a decade ago, after Concordia increased the price per pack by almost 6,000 per cent from 4.46 pounds to 258.19 pounds.

CMA Chief Executive Andrea Coscelli said: "Pharmaceutical companies which abuse their position and overcharge for drugs are forcing the NHS - and the U.K. taxpayer - to pay over the odds for important medical treatments.

"We allege that Concordia used its market dominance in the supply of liothyronine tablets to do exactly that."

The CMA has been targeting drugs companies over the prices they charge for certain drugs. The regulator can fine a company up to 10 percent of its annual worldwide group turnover for breaking competition law.

It recently fined Pfizer and Flynn Pharma nearly 90 million pounds over the price of anti-epilepsy treatment, phenytoin sodium capsules, and it has fined a number of companies a total of 45 million pounds over anti-depressant medicine paroxetine.

Both decisions are currently under appeal.

Liothyronine tablets are primarily used to treat hypothyroidism, a condition caused by a deficiency of thyroid hormone affecting at least 2 in every 100 people.

The CMA said it would consider representations from Concordia, and also Cinven and HgCapital - private equity firms and previous owners of entities now forming part of Concordia - before deciding whether competition law had in fact been broken.

Concordia said it did not believe that competition law had been infringed.

"The pricing of liothyronine has been conducted openly and transparently with the Department of Health in the U.K. over a period of 10 years," it said.

"Over that time, significant investment has been made in this medicine to ensure its continued availability for patients in the UK, to the specifications required by the Medicines and Healthcare products Regulatory Agency in the U.K.."

It said it would continue to cooperate with the CMA in its investigation.

The NHS advised in July that the drug should not be routinely prescribed due to its significant costs and limited evidence that it was preferable to an alternative.