The regulator said the companies were responsible for a significant rise in the price of the tablets in the UK between 2013 and 2016.
The price per pack increased from £49 to £88 over the period, which the CMA said deprived thousands of patients and raised costs for the NHS.
The tablets are used as the primary replacement treatment for people whose adrenal glands produce low amounts of natural steroid hormones.
In particular, Actavis UK has been accused of abusing its dominant position with regards to the hydrocortisone tablets.
CMA senior responsible officer Andrew Groves said: “We allege these agreements were intended to keep Actavis UK as the sole supplier of a drug relied on by thousands of patients – and in a position which could allow it to dictate and prolong high prices.
“As always at this stage in an investigation, these findings are provisional and no conclusion should be drawn at this stage that there has in fact been any breach of competition law. We will carefully consider any representations of the companies under investigation before determining whether the law has been infringed.”
Under the agreements deemed to be anti-competitive by the CMA, Actavis UK had delivered Concordia with a fixed supply of its own 10mg tablets at low price for the latter to resell them to customers in the UK.
A different investigation from the watchdog in December last year had accused Actavis UK of charging unreasonably high prices to the NHS for the tablets after hiking them by 12,000% through a number of years.
The anti-competition watchdog had concluded that the agreements allowed Actavis UK to extend the excessive prices in the market.
It also accused Actavis UK for not allowing the NHS get considerable price reductions that would have otherwise resulted from generic versions of the hydrocortisone tablets from its competitors.
Image: CMA alleged that Actavis UK and Concordia made illegal deals on hydrocortisone tablets. Photo: courtesy of Crown copyright.