The deal includes a $1.6-billion fee if AbbVie tries to walk away.
Shire and AbbVie's courtship has entered round five. A deal would offer AbbVie tax relief, and possibly make it an acquisition target.
Abbott and Mylan are setting up a generics business. The all-stock deal will give some branded generics a new home and offer Abbott a tax benefit that will increase over time.
Salix Pharmaceuticals joins the tax-inversion migration by purchasing Cosmo Technologies.