Pfizer has agreed a $3.6bn definitive merger acquisition deal with King Pharmaceuticals.

Pfizer will commence with a cash tender offer for all outstanding shares in King’s common stock, with the deal to be finalised in the fourth quarter of 2010 or the first quarter of 2011.

However, the deal has come under scrutiny form the Kendall Law Group who are investigating shareholders connected with the acquisition.

The national securities firm has said it is seeking to determine whether King Pharmaceuticals and its Board breached their fiduciary duties by entering into the agreement without considering better value deals for shareholders.

Once complete, Pfizer will gain control of significant stake in the pain relief market.