(Reuters) - Shares of companies owning pharmacy benefit managers fell on Wednesday after the introduction of a bipartisan bill that would force health insurers or drug middlemen to divest their pharmacy businesses.

CVS Health's Caremark, Cigna's Express Scripts and UnitedHealth Group's Optum control the majority of pharmacy benefit management (PBMs) in the US, while their parent companies also operate health insurance businesses.

Shares of all three companies were down between 4.8% to 5.5% after the Wall Street Journal first reported news of the bill.

The bill, sponsored by U.S. Senators Elizabeth Warren, a Democrat, and Josh Hawley, a Republican, will force companies owning health insurers or pharmacy benefit managers to divest their businesses operating pharmacies within three years.

Representatives Diana Harshbarger, a Republican, and Jake Auchincloss, a Democrat, are also supporting the bill, which will be introduced in the Congress.

PBMs negotiate prescription drug prices between insurers, pharmacies and drugmakers, and directly reimburse pharmacies for prescription drugs included under their agreed terms.

They have previously come under scrutiny for their influence over prescription drug prices.

"PBMs have manipulated the market to enrich themselves - hiking up drug costs, cheating employers, and driving small pharmacies out of business. My new bipartisan bill will untangle these conflicts of interest by reining in these middlemen," said Senator Warren.

Shares of other insurers such as Elevance, Humana and Centene fell between 1% and 3%.

"The latest introduction of potential legislation to restrict PBM operations and broader healthcare vertical integration is unlikely to gain traction, although it is hard to dismiss outright," said Leerink Partners analyst Michael Cherny.

Shares of insurers have come under pressure after Brian Thompson, the CEO of UnitedHealth's health insurance unit, was fatally shot outside a Manhattan hotel last week.