AstraZeneca shares fell more than 4% to $162.13 while Bristol Myers rose nearly 6% to $69.20 after weekend reports said the two drugmakers had held merger talks.
Analysts called a deal unlikely because the companies overlap in several therapies, especially lung cancer, where BMS's Opdivo posted $10.05 billion in 2025 sales and AstraZeneca's Imfinzi generated $6.06 billion.
BMO also said neither company has the balance-sheet capacity to buy the other outright, estimating deal firepower at about $32 billion for BMS and $37 billion for AstraZeneca versus market values above $130 billion each.
If regulators and financing hurdles were somehow cleared, Jefferies said the tie-up would create a roughly $100 billion-sales oncology leader and could spur a broader wave of large-cap pharma M&A.