AstraZeneca had a rough start to the week. On Monday, its London-listed shares sank as much as 7% at one point. They were still down 4.7% at last check. The trigger was a Financial Times report saying the drugmaker had talked with Bristol Myers Squibb about a possible merger.
Both drugmakers stayed silent on the rumor. AstraZeneca refused to issue a statement, and Bristol Myers Squibb didn't answer CNBC's inquiry outside regular U.S. trading hours. That did not stop the rumor from moving markets on both sides of the Atlantic.
Bristol Myers, based in Princeton, New Jersey, saw the same headline land differently. Its shares jumped 6% in premarket trading. Were it to go through, the combined entity would be worth around $400 billion, making it one of the biggest pharma acquisitions in history.
Heading into Monday, AstraZeneca's market capitalization stood at $264 billion. Bristol Myers' market cap is roughly $133 billion.
Jefferies analysts were honest about their reaction. "Given the strength of AZ's growth and innovation profile, we are a bit perplexed," they wrote.
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They added that "if there is one company that doesn't need financial engineering, it's AZ."
The company has set a revenue goal of $80 billion for 2030, compared with $58.7 billion in the prior year. Pascal Soriot has run the company since 2012. The CFO told CNBC he is "very confident we will hit $80 billion revenue target."
Bristol Myers, by contrast, has several drugs approaching patent expiry. The company is projected to see growth taper off starting next year as patent protections lapse and generic versions of blockbuster medicines Eliquis and Opdivo enter the market.
Citi analysts called the reported talks a "surprise" if true, and they described AstraZeneca's pipeline as "best-in-class." RBC Capital Markets analysts pointed to uncertain pipeline synergies and upcoming trial results for Bristol Myers' drugs milvexian and Cobenfy.
An experimental AstraZeneca heart-disease medicine missed its primary goal in a Phase 3 trial earlier this month, prompting fresh questions about the credibility of management.
Jefferies expects deal talks to center on creating a massive cancer-drug franchise, and the merged companies' oncology lineup would probably be the industry's most extensive, which could draw regulatory review.
Both companies lean heavily on the United States. AstraZeneca got 42% of its sales from the U.S. in the first half of 2026. Bristol Myers pulled 69% of its revenue from the U.S. in its latest quarter. Earlier this year, AstraZeneca shifted from an ADR structure to a full direct listing on the NYSE.
The timing of the report also struck analysts as strange, and they had questions about why the two companies would consider this.
Because AstraZeneca carries so much weight in the index, its slide dragged on Britain's FTSE 100 benchmark, which ended the session roughly unchanged. A single unconfirmed story was enough to move two giant drug companies in opposite directions.
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