Merger Madness: How Much Is Enough?

It's been no secret that Pfizer was planning something big, in an already blockbuster year for M&A. Yet somehow the durability of this year's streak of dealmaking and the money at stake still boggles the mind.

Deal mania first surfaced in early 2014, with an increasing desire by U.S. companies to use large, complex deals as a way to flee stifling taxes, the latest method of boosting their stagnating earnings. This was quickly met with resistance from the Treasury Department, but in government fashion, the Treasury dragged its feet in firming up the actual rules to prevent tax-inversion deals. It had success in stopping some big companies from packing their bags for the greener pastures of Ireland, but these types of transactions still persist.

Now, the biggest inversion of them all may be en route: Pfizer, the 12th-largest American corporation, is in talks to buy Allergan, the $120 billion maker of Botox wrinkle injections that's based, where else, in Dublin.

The mega-merger engine keeps chugging along despite forces that could have easily derailed it -- a bumpy stock market, a slowdown in emerging economies, controversy surrounding the pharmaceutical industry's pricing methods, etc. Super-sized deals and tax-dodging practices have even become talking points for presidential candidates.