Pfizer Inc. PFE and Allergan Inc. AGN are considering a merger that would create a drugs giant worth over $300 billion, according to The Wall Street Journal and The Financial Times.

If it comes off, the merger, would top off a blockbuster year for M&A in the pharmaceuticals sector this year that has seen over $800 billion worth of deals. Today’s Allergan, with a market capitalization of $112 billion, is itself the product of five major deals since 2012, including, only 11 months ago, a $66 billion deal with Actavis (read more about Allergan’s deal-driven evolution here). Pfizer’s market cap is around $217 billion. Together, they would eclipse Johnson & Johnson JNJ as the world’s largest healthcare company.

The pharma sector’s merger frenzy has been driven as much as by financial engineering as by the search for scale in a U.S. healthcare market rapidly evolving under the influence of the Affordable Care Act. One of the chief attractions of Allergan for Pfizer is its domicile in Ireland–the FT noted that Allergan had an effective tax rate of 4.8%, compared to Pfizer’s 25.5%. Pfizer had previously tried to shift its tax domicile abroad last year by buying U.K.-based AstraZeneca, but political and investor concerns scuppered the deal.

The WSJ reported that Pfizer, the maker of erectile dysfunction drug Viagra and Lipitor, once the world’s best-selling statin, approached Allergan recently, but said the process is still at an early stage and may not yield an agreement. Allergan’s best-known product is anti-wrinkle treatment Botox.