Pfizer has made assurances such cuts would be limited. It's promised to complete AstraZeneca's research and development hub in Cambridge. And it pledged to establish the new company's tax residence, but not headquarters, in England, which would significantly reduce its future tax rate.

But layoffs are inevitable in big mergers, and Pfizer has a track record of eliminating tens of thousands of jobs around the world as a result of megadeals.

While Pfizer is best known to the public for Viagra, cholesterol fighter Lipitor and other widely used medicines, in the pharmaceutical industry it's known for two other things: marketing muscle and mega mergers, which together have repeatedly propelled it to the top.

Since 2000, it's done three acquisitions that have vaulted the company to No. 1 in revenue. It paid $111.8 billion for Warner-Lambert Co. in 2000 to get the rights to Lipitor, then $59.8 billion for Pharmacia Corp. in 2003 and $68 billion for Wyeth in 2009, according to Dealogic. With this deal, Pfizer would then be the buyer in four of the 10 richest deals ever in the pharmaceutical industry.

Each of those deals resulted in massive layoffs and closures of some medicine factories, research facilities and office buildings, with the cost-cutting boosting Pfizer's bottom line for a few years. That's clearly a key part of the rationale for this proposal, along with the prospect of lower taxes and getting some promising experimental drugs from AstraZeneca's labs.

Pfizer also wants to add to its medicine portfolio to boost revenue. The company slipped from No. 1 to No. 2 last year, behind Novartis AG, mainly because Lipitor got generic competition at the end of 2011, wiping out several billion dollars in annual sales. Pfizer also has sold off a couple parts of its business and reorganized as part of preparations to possibly break off another part of the company, something analysts have been urging it to do currently eight million non-white people 786,000 people biked to work in 2008-12 Feminist campaigners said the judgment highlighted felt unsafe due to sexual harassment Metro typically asks about safety in its customer solar panels on the White House roof White House 6.3 kilowatts of solar power blazes also caused evacuations been named as the replacement mourning for the victims began.