COPENHAGEN/LONDON, May 7 (Reuters) - Obesity drugmaker Novo Nordisk downgraded its 2025 sales forecast on Wednesday, ending a four-year stretch of steady guidance increases since the launch of its Wegovy weight-loss drug, as U.S. prescriptions ceased to grow.

The company said first-quarter sales of its first-to-market weight-loss drug Wegovy were 17.36 billion Danish crowns ($2.64 billion), declining 13% from the previous quarter, and below the 18.7 billion crowns expected by analysts.

The downgrade and Wegovy sales miss are likely to deepen concerns among investors that Novo is losing its lead in the fiercely competitive obesity drug market to chief rival Eli Lilly.

US-listed shares of Novo Nordisk rallied 4.5% in overnight trading, erasing earlier losses​.

Booming sales of Wegovy helped to make Novo Europe's most valuable listed company, worth $615 billion at its peak.

But prescriptions in the United States, its biggest market, have not grown since February despite Novo having increased supplies of Wegovy to meet demand. Novo's market value has halved to about $310 billion.

The Danish company said it expects 2025 sales growth in local currencies of between 13% and 21%, compared to the 16%-24% range given at the beginning of the year.

It also predicts operating profit growth between 16% and 24% this year, compared to a previously guided range of between 19% and 27%.

Analysts forecast sales and operating profit this year will grow by 17.8% and 21.5%, respectively, according to a company-compiled consensus based on 26 analysts.

"In the first quarter of 2025, we delivered 18% sales growth and continued to expand the reach of our innovative GLP-1 treatments," CEO Lars Fruergaard Jorgensen said in a statement.

"However, we have reduced our full-year outlook due to lower than-planned branded GLP-1 penetration, which is impacted by the rapid expansion of compounding in the U.S.," he said.

Novo faces competition from Eli Lilly's Zepbound obesity shot, whose U.S. prescriptions surpass Wegovy.

Novo reported first-quarter earnings before interest and taxation of 38.79 billion crowns, compared with the 37.20 billion forecast in a company-compiled consensus based on 26 analysts and up 22% from a year ago.

($1 = 6.5675 Danish crowns)