CVS Health Corp.'s $68 billion merger with Aetna Inc. could stop emerging pharmacy benefit services from entering an already consolidated market, industry participants and observers told Bloomberg Law.

The deal cleared federal antitrust regulators Oct. 10, with Justice Department antitrust chief Makan Delrahim touting the vertical integration of a pharmacy and health benefits company as a bonus for consumers. The DOJ cleared a similar deal in Cigna Corp.'s $54 billion takeover over Express Scripts Holding Co. in September.

Critics say...