AstraZeneca and Bristol Myers Squibb confirmed on Tuesday that they were in preliminary talks over a possible combination, a deal that would rank among the largest in pharmaceutical history if completed. The companies cautioned that no agreement had been reached and that discussions could still collapse.

The talks come as both drugmakers confront looming patent expirations on key medicines. AstraZeneca, based in Cambridge, England, faces competition to its oncology franchise later this decade, while New York-listed Bristol Myers Squibb is bracing for the loss of exclusivity on its blood thinner Eliquis and cancer therapy Opdivo.

Analysts at Jefferies and Leerink Partners said a combination would create a formidable oncology and cardiovascular portfolio. The deal would draw intense scrutiny from competition regulators in the United States, the European Union and the United Kingdom. AstraZeneca chief executive Pascal Soriot has repeatedly signalled ambitions to expand the company's reach.

Shares in both companies moved sharply after the disclosure, with investors weighing the strategic logic against the regulatory hurdles and financing costs of a transaction that could exceed $100 billion. A spokesperson for AstraZeneca declined to comment beyond the confirmation, while Bristol Myers Squibb said it did not discuss market speculation.

Any agreement would require approval from shareholders and antitrust authorities, a process that could stretch well into 2027. Recent large pharmaceutical mergers have faced prolonged reviews over concerns about pricing and pipeline overlap.