Walgreen Co. will pay $6.7 billion in cash and stock to buy a stake in European health and beauty retailer Alliance Boots in the first international venture for the largest U.S. drugstore chain.

The Deerfield, Ill., company said today it plans to initially spend about $4 billion in cash and contribute more than 83 million shares for a 45 percent ownership stake in Switzerland-based Alliance Boots, which runs more than 3,300 health and beauty retail stores in 11 countries.

Walgreen has an option to buy the rest of the company in about three years in a deal valued at $9.5 billion based on Walgreen’s share price and the current exchange rate.

The acquisition comes while Walgreen works through a sales slump prompted largely by a split with pharmacy benefits manager Express Scripts Holding Co. Walgreen has seen sales fall for several months since it stopped filling prescriptions for Express Scripts, the largest U.S. pharmacy benefits manager, at the end of last year.