Drug companies accused of charging "excessive prices" to the NHS are under investigation, the competition watchdog has announced.
The Competition and Markets Authority (CMA) launched an investigation into suspected breaches of competition law by pharmaceutical firms on Tuesday.
If the companies are found to have broken the law they face fines of up to 10% of their turnover.
The CMA said: "The investigation relates to suspected unfair pricing by way of charging excessive prices in the supply of certain pharmaceutical products, including to the National Health Service."
Health Secretary Jeremy Hunt called in the CMA in June after an investigation by The Times suggested companies were exploiting a loophole in NHS rules to raise prices of medicines.
The newspaper alleged that companies face limited competition on long-established, off-patent drugs, bought from large pharmaceutical firms.
It was also claimed that the prices of 32 drugs have risen by more than 1,000% in the past five years.
Concordia International said it was one of the companies being investigated.
A spokesman said: "We are working co-operatively to better understand the CMA's position and we will continue to work constructively to resolve the matter.
"Although Concordia has also had past discussions with the CMA regarding the supply of certain of its products in the UK, this is the first interaction with the CMA regarding the company's pricing."