Walgreen, the biggest US drugstore chain, agreed to buy a 45 percent stake in Alliance Boots for $6.7 billion in an attempt to create a global chain of pharmacies.

Walgreen will pay $4 billion cash and the rest in stock for its biggest-ever deal, with an option to gain full control in about three years, the companies said.

The combination gives Walgreen and Alliance Boots — the United Kingdom’s largest drugstore-chain — more than 11,000 stores in 12 countries.