Adds context on pharmaceutical advertising in paragraph 7 and on lawsuits in paragraph 10, updates shares in paragraph 12

By Maggie Fick and Michael Erman

LONDON, July 21 (Reuters) - Novo Nordisk NOVOb.CO sued rival Eli Lilly LLY.N in a U.S. federal court on Tuesday, accusing the U.S. drugmaker of false advertising in claiming its weight-loss medicines outperform Novo's drugs.

The Danish company filed the lawsuit in U.S. District Court in New Jersey, alleging Lilly violated federal and state false advertising and unfair competition laws, including the Lanham Act, through nationwide advertising campaigns for obesity drug Zepbound and diabetes treatment Mounjaro.

Novo alleges Lilly compared the highest approved doses of its medicines with lower doses of Novo's Wegovy and Ozempic while omitting newer, higher-dose versions that Novo says deliver greater weight loss.

Indianapolis-based Eli Lilly did not immediately respond to a request for comment.

"The ads are maliciously and deceptively false because Lilly knowingly cites outdated clinical trials that compare the highest doses of the Lilly medicines to lower doses of Novo Nordisk’s medicines," Novo wrote in its complaint.

Novo and Lilly are locked in a fierce battle for dominance in the U.S. obesity drug market, which analysts expect to be worth more than $100 billion by 2030.

The U.S. is one of only a handful of countries that allow pharmaceutical companies to advertise prescription medicines directly to consumers. Drugmakers spend billions of dollars every year on television and other advertising.

Novo was first to market with weight-loss injection Wegovy, but later lost the lead to Lilly's rival Zepbound. After replacing its CEO and issuing multiple profit warnings last year, Novo has landed a counterpunch with oral Wegovy, whose strong demand has held up despite the launch of Lilly's competing pill.

Novo said it is seeking a court order requiring Lilly to withdraw the advertisements and run a corrective advertising campaign. It added that it plans to seek a preliminary injunction if Lilly does not voluntarily remove the ads.

Pharmaceutical companies regularly sue each other over patent infringement claims. False advertising suits are less common.

Novo is also seeking damages, including Lilly's profits that can be attributed to the ads.

Copenhagen-listed Novo Nordisk NOVOb.CO shares dropped 2% in afternoon trading, while Lilly shares rose marginally, up 0.5%.

'INADEQUATE DISCLAIMER' IN ADS

Novo said it sent Lilly a cease-and-desist letter in April after the U.S. approved a 7.2-milligram dose of Wegovy. Lilly did not respond to the letter, Novo's general counsel John Kuckelman told Reuters, adding that the company instead inserted what he described as an inadequate disclaimer into the advertisements.

Novo says the scale of Lilly's campaign is massive. The ad campaign has been served up to consumers more than 700 million times just since Lilly modified its ad after Novo's letter in late April.

Kuckelman said Lilly's advertisements compare weight-loss results of about 50 pounds for Zepbound with about 33 pounds for Wegovy, even though no head-to-head trial has compared the highest approved doses of the medicines. He said separate late-stage trials of the highest doses showed average weight loss of about 48 pounds for Zepbound and 47 pounds for Wegovy.