Baird analyst Brian Skorney downgraded Amgen to Underperform from Neutral with a price target of $185, up from $173. The stock is trading down 4% to $217.90 in premarket trading following last night's Q4 results.

Amgen is a "great company and the strong execution in the face of some serious headwinds has been more than reflected in the stock," Skorney tells investors in a post-earnings research note.

He believes the stock's "current premium reflects a level of enthusiasm that just isn't warranted" given the company's stage of maturity. Skorney believes the "exuberance is overdone" and is calling for a pullback in Amgen shares.