Allergan's stock is trading around $165, having jumped 15 percent on news of Valeant's offer.
With Shire, Allergan would gain the ADHD drug Vyvanse, which drew $1.23 billion in 2013 revenue. Shire also focuses on rare diseases, with drugs like Elaprase for an enzyme deficiency called Hunter syndrome and Replagal for Fabry disease drawing $546 million and $468 million last year, respectively.
And the 12.5 percent Irish corporate tax rate can't be ignored—despite Pyott's comments earlier this year that he's unlikely to step in line with industry trends and seek acquisitions there.