Allergan (NYSE: AGN) continues to fight off the advances of serial acquirer Valeant (NYSE: VRX) and its partner, hedge fund manager Bill Ackman. Ackman's Pershing Square is Allergan's largest shareholder, and is hoping to convert enough other investors to his side as this deal has gone hostile. Given Allergan's tenous position, rumors continue to swirl that it will make a play for Irish drugmaker Shire (Nasdaq: SHPG).  But given the lure of Ireland's low tax rate, Allergan may have to get into a bidding war with AbbVie to pull off its own deal for the ADHD drugmaker.

In this episode of Market Check-Up, the otley Fool's health care focused investing show, analysts David WIlliamson and Michael Douglass, discuss the agressive new posture of Ackman and Valeant, how Allergan is responding, and why its only defense may be to go on the offensive.