AstraZeneca Plc (NYSE ADR: AZN) stock dropped as much as 15% today (Monday) following news that the British pharmaceutical company had rejected Pfizer Inc.'s (NYSE: PFE) final takeover offer. That's the largest intra-day loss in AZN stock's history.

Pfizer upped its offer to $118 billion on Sunday, which was up from the $106 billion it had offered in early May. A merger between the two companies would have created the world's largest pharmaceutical group.

When Pfizer officials made the $118 billion offer, they also stated that this was their final attempt at a merger. AstraZeneca Chairman Leif Johansson told Reuters that he doesn't see any scenario in which the deal would be completed by the May 26 deadline that has been set under U.K. takeover rules. He also said he doesn't see the deadline being extended.

With the merger, Pfizer would have been able to create a tax base in the U.K. Currently, Pfizer pays a corporate tax rate of 27% in the United States. The corporate tax rate in the U.K. is expected to soon drop to 20%.

However, officials from AstraZeneca feared a merger would lead to job cuts and a lessened focus on research. According to Reuters, numerous British scientists and politicians opposed the deal, saying it would undermine Great Britain's science base.

"Pfizer's approach throughout its pursuit of AstraZeneca appears to have been fundamentally driven by the corporate financial benefits to its shareholders of cost savings and tax minimization," a statement from Johansson said. "From our first meeting in January to our latest discussion yesterday, and in the numerous phone calls in between, Pfizer has failed to make a compelling strategic, business or value case."

Clearly, shareholders did not hold the same sentiment, as shares of AZN tumbled in morning trading. AZN stock had been hovering near an all-time high of $82.68 during the month of May. Today, Shares dropped as low as $69.78 on the news.

Pfizer officials said that they would not pursue a hostile takeover. PFE stock was largely unaffected by the news, trading up 1% on Monday afternoon.

Now that a deal between Pfizer and AstraZeneca appears dead in the water, here's what investors can expect moving forward...

Under British takeover rules, the companies have until noon on May 26, to complete a deal. Once that deadline passes, Pfizer will have to wait six months until it is allowed to make another offer.

While Pfizer cannot make another offer, AstraZeneca can counteroffer after three months have passed. That, however, appears unlikely.

"AstraZeneca will have six months to demonstrate that it was right to reject Pfizer's offer, or face the prospect of a fresh approach," Edison Investment Research Analyst Mick Cooper told Reuters.

It's difficult to imagine Pfizer sitting by idly and waiting the full six months without exploring other takeover targets in the U.K. or other parts of Europe. But another takeover may be a small consolation, as AstraZeneca would have brought more than just tax breaks to Pfizer.

Following Pfizer's previous bid, AZN officials released financial outlook figures to illustrate why they don't need PFE. At the time, the company projected annual revenue to increase by 84% by 2023. AZN also said it expects sales to reach $84 billion per year by that time, which is up from $25.71 billion in 2013.

AstraZeneca estimates that annual sales from its upcoming drugs could be between $23 billion and $63 billion, depending on which receive approval. AZN expects its diabetes and respiratory divisions to each account for $8 billion in revenue annually by 2023.

That deep pipeline would have helped PFE boost sales, which dropped 6% in 2013. AZN's sales also dropped 6% in 2013, but the company is clearly optimistic about its upcoming drug pipeline.

If AstraZeneca holds strong in its resistance to Pfizer's takeover, pressure from shareholders will likely mount. They aren't pleased with today's news, as evidenced by the largest ever one-day drop for the stock, and some have expressed discontent.

"We are disappointed the board of AstraZeneca has rejected Pfizer's latest offer so categorically," Jupiter Distribution Fund Co-Manager Alastair Gunn told Bloomberg. "They should have at least engaged in a constructive conversation with Pfizer on the details of the offer to assess the opportunities that a combined entity could bring."

There's still a chance a last-minute deal could be struck if enough pressure mounts, but the odds are slim.

"There's still a chance Astra shareholders will put pressure on management and try to push for a compromise, but the chances of this deal not being agreed on just increased," Panmure Gordon & Co. Analyst Savvas Neophytou told Bloomberg.

What do you think a deal between Pfizer and AstraZeneca would mean for PFE and AZN stock? Join the conversation on Twitter @moneymorning using #AZN or $AZN.

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Related Articles:

• Reuters: AstraZeneca Rejects Pfizer's Take-It-Or-Leave-It Offer
• Bloomberg: AstraZeneca Rejects Pfizer's $117 Billion Offer as Too Low