Pfizer is buying weight-loss drugmaker Metsera. Why Novo Nordisk is the real winner.

Metsera shares tumbled Monday after Pfizer agreed to buy the weight-loss drug start-up in a deal that could value it at more than $10 billion.

Wegovy and Ozempic maker Novo Nordisk lost this unusual bidding war but could be the overall winner of the deal.

Metsera shares fell 15% to $70.88 before the US open, while Pfizer rose 0.5% to $24.56. Novo’s Copenhagen-listed shares rose 2.1%.

The moves come after Metsera announced late Friday that it had accepted an offer from Pfizer of $65.60 per share and a contingent valuation right that entitles Metsera shareholders to additional payments of up to $20.65 per share if certain milestones are reached.

Pfizer agreed to buy Metsera for $7.3 billion in September; Novo responded with a $9 billion bid of its own, sparking a bidding war.

Market reaction on Monday suggests Novo investors are relieved that the Danish drugmaker did not match Pfizer’s latest offer; This seems like an overpayment, given that Metsera’s treatments are largely unproven. Novo said in a statement that it would not increase its offer due to its commitment to financial discipline and shareholder value.

Pfizer said it expects to complete the deal after a meeting of Metsera shareholders on Thursday.

Write to George Glover at george.glover@dowjones.com