Pfizer will purchase biotech firm Medivation, which specializes in cancer treatments, for some $14 billion, the US pharma giant said in a statement on Monday.

Pfizer's offer for the San Francisco-based Medivation goes far beyond the company's market value of $11.1 billion.

The boards of directors of both companies have unanimously approved the merger, the statement said.

"The proposed acquisition of Medivation is expected to immediately accelerate revenue growth and drive overall earnings growth potential for Pfizer," said Ian Read, Pfizer chairman and chief executive officer.