Astrazeneca share price tumbles on $400bn megamerger talksAstrazeneca shareholders have questioned the possible mergerAstrazeneca’s share price slumped sharply on Monday as investors baulked at a potential $400bn megamerger with US rival Bristol Myers Squibb. But investors dumped shares in Astrazeneca yesterday morning in a sign of concern over the potential cost and structure of the deal. Both Astrazeneca and Bristol Myers Squibb have an “overlapping focus” in oncology leading to substantial antitrust scrutiny, particularly in the US. Oncology drugs also accounted for more than 40 per cent of Bristol Myers Squibb’s overall sales in the first six months of 2026. “Political pressure might also be brought to bear, particularly on these shores given the importance of AstraZeneca to UK plc.”