Novo Nordisk ( NVO 0.33%) stock jumped 3.1% at the open Thursday morning before drifting back. As of 10:54 a.m. ET, it was still up by about 1.2%.

The U.K.'s FTSE 100 moved between losses and gains on Thursday, with most other European markets closed for the May 1 holiday.The FTSE 100 was flat at 12:34 p.m. in London after notching its 13th straight positive session in its longest winning run since late 2016 into early 2017.

The deal could expand access to its in-demand obesity drug at the expense of Eli Lilly, whose shares fell by double digits on Thursday.