Drugstore operator Walgreen Co. formally ended a strained alliance with Theranos Inc. as regulators near a decision on whether to impose sanctions against the embattled Silicon Valley firm.

Some officials at the Walgreens Boots Alliance Inc. unit had grown frustrated at not getting more details and documentation from Theranos after learning it had corrected tens of thousands of blood tests, including many performed on samples collected from patients at Walgreens pharmacies, according to people familiar with the partnership....