“For people with diabetes, insulin can be as essential as air,” said Dr. Steve Miller, the executive vice president and chief clinical officer of Cigna, which merged last year with Express Scripts. “We need to ensure these individuals feel secure in their ability to afford every fill so they don’t miss one dose, which can be dangerous for their health.”

Multiple congressional inquiries have focused on insulin, and on Tuesday, a subcommittee of the House Energy and Commerce Committee held a hearing on insulin prices that featured testimony from patients and doctors. Executives from the top pharmacy benefit managers, including Express Scripts, are scheduled to testify before the Senate Finance Committee next week, when the price of insulin will also likely come up.

Senators Charles Grassley, Republican of Iowa and the finance panel’s chairman, and Ron Wyden, the panel’s ranking Democrat from Oregon, sent letters to Express Scripts and two other major drug benefit managers asking for more information about the role they play in rising insulin costs. "The essential question is whether the practices employed by P.B.M.s actually reduce the cost of insulin for patients and achieve the lowest possible federal spending," the senators said in their letter.

Under the new plan, employers who cover their workers through Cigna and Express Scripts can opt into the program. The extra costs will be picked up by the three drug makers that sell insulin — Eli Lilly, Novo Nordisk and Sanofi. Because the manufacturers are covering the costs, anti-kickback laws prohibit Medicare beneficiaries and other people insured through government health care programs from these types of negotiated arrangements.

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Cigna and Express Scripts said the average out-of-pocket cost for consumers under their existing plans was $41.50 for a 30-day supply of insulin. (Another large pharmacy benefit manager, OptumRx, which is owned by UnitedHealthcare, cited similar costs.) The companies said that patients with a high deductible or coinsurance — where they must pay a percentage of a drug’s list price — would benefit the most.