Drugstore chain Walgreen Co. is buying an ownership stake in another big company, this time in a deal that could increase its clout when negotiating prices with drugmakers.

The Deerfield, Ill., company said Tuesday that it is expanding its supply agreement with AmerisourceBergen Corp. through a 10-year deal that gives it and European health and beauty retailer Alliance Boots a minority stake in the pharmaceutical wholesaler. Walgreen has a stake in Alliance Boots.