AstraZeneca and Bristol Myers Squibb terminated their proposed $400bn merger after AstraZeneca's shares dropped 9% on August 3, marking the pharma group's worst trading day since the pandemic. The deal, which would have been one of the largest in pharmaceutical history, collapsed amid investor revolt and concerns about the strategic logic of combining two major drugmakers.

The two companies had been in serious talks since spring, with discussions focusing on a share-based transaction where AstraZeneca would acquire BMS at a premium. By early August, they were nearing agreement on price and consulted US banks about financing the multibillion-dollar cash component.

Investors questioned the merger's rationale, particularly given BMS's impending patent expirations that could halve its revenues. One shareholder noted the deal felt like AstraZeneca was 'trying to get out of England,' while others worried about antitrust scrutiny given both companies' large oncology divisions. Politics also played a role, with UK officials concerned about a British pharmaceutical crown jewel shifting to US control.