Logic of a pharma megamerger is questionable, but AstraZeneca's record isn't. Few big-pharma bosses enjoy a stronger reputation than Pascal Soriot. The idea he would stake the company's future on a takeover of another pharma giant left investors scratching heads, with many running for cover.

After plunging on a Financial Times report of merger discussions with Bristol-Myers Squibb, shares recovered when Reuters reported no ongoing talks. Either way, shares are down over 10% for the year, trading at less than 15 times forward earnings, well below the 18-times decade average.

Soriot built his reputation by doing the opposite of what he might now contemplate. After rebuffing Pfizer's $120 billion bid in 2014, he transformed AstraZeneca into a top growth story via internal R&D and smaller deals, like the $39 billion Alexion acquisition. Revenue rose to nearly $60 billion from $25 billion.

But could Soriot harbor larger ambitions? At roughly $250 billion market cap, AstraZeneca is dwarfed by Eli Lilly (~$1 trillion) and Johnson & Johnson (~$600 billion). That gap explains why he might consider a megamerger creating a $400 billion global pharma company.