Another part of Pfizer's rationale for the bid is the tax savings which could result, both from avoiding tax on overseas revenues, and by incorporating a merged company in the U.K.. This part of the story has led to a debate over U.S. tax rules, with magnates like former General Electric chief executive Jack Welch weighing in over U.S. tax policy.
Read MoreJack Welch: U.S. doesn't have rational tax policy
AstraZeneca's share price, which has shot up by 23 percent since Pfizer's initial approach was reported by the U.K.'s Sunday Times less than a fortnight ago, would likely be deflated if a bid failed to emerge.
Read MoreAstraZeneca shares soar on Pfizer interest
Analysts have long considered the U.K. company as vulnerable to a takeover approach, after a difficult few years with patents expiring on key drugs and unrest among shareholders over executive pay.
"The strategic logic and financial uncertainty around AstraZeneca's ability to deliver on its strategy alone make a deal more than likely," according to pharma analysts at Credit Suisse.
Pfizer is being advised by Bank of America/Merrill Lynch, Guggenheim Securities, and JPMorgan Chase, while AstraZeneca's advisers are Robey Warshaw, Evercore Partners, Goldman Sachs, and Morgan Stanley.
—By CNBC's Catherine Boyle