The pharmacy benefit business at Minnetonka-based UnitedHealth Group is partnering with Walgreens to give patients 90-day prescriptions with low copays in retail stores starting next year.

The move is important for United's OptumRx unit, because some patients who like shopping at drugstores want prescriptions at the quantity and cost typically found with mail-order, said Mark Thierer, the division's chief executive.

OptumRx is the nation's third largest pharmaceutical benefit manager behind St. Louis-based Express Scripts and Rhode Island-based CVS Caremark. The rivals have provided low-copay access to 90-day supplies in retail pharmacies for years.

"Eight out of 10 pills that get dispensed in the United States are dispensed through a retail channel," Thierer said in an interview. "Patients like walking into a retail pharmacy for many of the medications that they utilize."

Financial terms weren't disclosed, and some stock analysts called the deal immaterial.

UnitedHealth Group operates UnitedHealthcare, which is the nation's largest health insurer, as well as Eden Prairie-based Optum, which is the company's health services division.

Optum's prescription management business grew considerably last year with the $12.8 billion acquisition of an Illinois-based pharmaceutical benefit manager (PBM) called Catamaran.

Insurers hire PBMs to negotiate prices with drug companies and help structure formularies that can steer patients to lower-cost medications. PBMs also assemble networks of retail and mail-order pharmacies.

OptumRx operates five mail-order pharmacies across the country. For years, health plans and their PBM partners have offered three-month medicine supplies to patients with lower copays, so long as drugs were delivered to patients' homes.

The new program will be an option for employer groups starting next year. It's a first step in a broader partnership between OptumRx and Walgreens Boots Alliance, Inc. Another initiative would give Walgreens pharmacists better information for managing a patient's overall medication therapy.

Express Scripts announced a 90-day retail program with Illinois-based Walgreens in 2013, while CVS Caremark said it's provided three-month supplies at the company's CVS pharmacies since 2009.

Express Scripts managed 1.3 billion prescriptions in 2015, when CVS managed about 1.2 billion. OptumRx expects to manage slightly more than 1 billion prescriptions this year, Thierer said.

"It has really become sort of a three-horse race," he said. "When you collect them all up, that does represent about 75 percent of the market."

Analysts have been watching the PBM race closely given a proposed $48 billion acquisition announced last year of health insurer Cigna by Anthem. Cigna uses OptumRx as its PBM, while Anthem uses Express Scripts.

Eagan-based Prime Therapeutics also competes in the PBM market, along with ClearScript, which is owned by Minneapolis-based Fairview Health Services.

Christopher Snowbeck • 612-673-4744

Twitter: @chrissnowbeck