The weight-loss drug wars reached a new level of intensity Tuesday as Novo Nordisk sued Eli Lilly, accusing its arch-rival of misleading advertising.Why it matters: Lilly and Novo are the market leaders in GLP-1 drug sales, controlling the lion's share of the surging segment.Zoom in: Novo — the maker of Ozempic and Wegovy — alleged Tuesday that Lilly has engaged in "a nationwide pattern of deceptive advertising which confuses consumers by using outdated studies" in its ads.The Danish drug giant said Lilly's ads compare the results from high-dosage GLP-1 treatments to lower dosages of Novo drugs that treat obesity and diabetes, with the dosage difference disclosed only in fine-print disclaimers."People deserve evidence-based, up-to-date information they can trust when making decisions about their health," Novo said in a statement. "Novo Nordisk has taken this step because Americans should not be denied the full picture of their treatment options."Reps for Lilly, which makes Zepbound and Mounjaro, did not immediately respond to a request for comment.State of play: The federal lawsuit comes amid a backdrop of growing scrutiny in Washington over drug advertising.Health and Human Services Secretary Robert F. Kennedy Jr. last year announced an FDA "crackdown" on drug ads.Sen. Josh Hawley (R-Mo.) and Sen. Jeanne Shaheen (D-NH) recently introduced a bipartisan bill to kill federal subsidies for pharmaceutical ads.Sen. Bernie Sanders (I-Vt.) and Sen. Angus King (I-Maine) recently introduced a bill to ban drug ads altogether.Zoom out: The Novo suit also comes as the GLP-1 competition grows increasingly competitive with the release of oral versions of the injectable drugs, which are already proving to be popular among the public.But if Lilly investors were concerned about the conflict, it didn't show in the stock market Tuesday. Lilly shares closed up 2.5% on the day, while Novo shares finished down 0.5% in New York.The bottom line: Novo wants a permanent injunction requiring Lilly to cease the ads on all platforms and to "conduct a corrective advertising campaign."