CVS Health and Aetna need to divest some of their Medicare Part D assets before the Department of Justice will grant approval for their proposed $69 billion mega-merger.
The DOJ is expected to conclude its review of the deal once the companies have satisfied the antitrust concern, Reuters has reported.
Aetna has a 9 percent market share among Part D plans while CVS Health has a 24 percent market share, with overlap in some markets.
In August, California Insurance Commissioner Dave Jones asked the DOJ to block the merger over Part D concerns, saying that reducing competition for the drug prescription plans would likely result in higher premiums.
One potential buyer for the Part D plans is WellCare Health Plans, according to The Wall Street Journal.
CVS has 6.1 million Part D members; UnitedHealth, 5.36 million; Humana 4.9 million; Express Scripts 2.5 million; and Aetna 2.2 million, WSJ reported.
Aetna already uses CVS Caremark for its pharmacy benefits but has retained ownership of PBM and clinical program management.
The expected approval for CVS Health to buy Aetna follows DOJ clearance this week of the $67 billion acquisition of Express Scripts by Cigna.
Prescription drugs prices are among the biggest factors in the rising cost of healthcare. For more than a year, legislators have questioned drug rebates and the current practice of PBMs not revealing the prices they negotiate with drug manufacturers, even to insurers.
UnitedHealth launched its integration with a PBM with its subsidiary, OptumRx. Last year, Anthem and CVS Health signed a five-year agreement to offer PBM Services through IngenioRx starting in 2020, coinciding with the end of Anthem's current contract with Express Scripts.
Approval for the vertical integration between payer and PBM has shown more success than the horizontal mergers proposed between Aetna, Humana and Anthem, Cigna. The DOJ blocked both mega bids on the basis of antitrust concerns.