Botox maker Allergan is buying Kythera Biopharmaceuticals for $2.1 billion, adding an injection that reduces "double chin" to its portfolio of products. The Dublin drugmaker will pay $75 per share in a cash-and-stock deal for Kythera, based in California, a 25.5 percent premium over its closing stock price Tuesday. Kythera Biopharmaceuticals ınc. is preparing to launch its double chin treatment Kybella in the U.S. and the company is seeking approval in foreign markets as well.