Walgreen Co. will pay $6.7 billion in cash and stock to buy a stake in European health and beauty retailer Alliance Boots, a deal that would give global clout to a U.S. drugstore chain struggling with slipping sales in its home market.

The combination would create the largest single purchaser of prescription drugs in the world and give Walgreen access to emerging markets like China and Egypt, company officials said Tuesday in announcing the deal. However, it also will plant the biggest U.S. drugstore chain in a continent beset by debt worries in Greece, Spain and other nations.

But Walgreen said Alliance Boots brings in revenue from some of the most stable countries in Europe. CEO Greg Wasson told analysts the acquisition is "a truly game changing step" for his Deerfield, Ill., company.