Novo Nordisk has sued Eli Lilly in a US federal court, alleging that advertising for Lilly’s weight-loss and diabetes medicines falsely suggests they are more effective than Novo’s rival products. The case escalates competition between the two leading makers of GLP-1 medicines, where advertising claims may influence patients, prescribers and a rapidly expanding market.

Novo says Lilly’s campaigns for Zepbound and Mounjaro compare their highest doses with lower doses of Wegovy and Ozempic, while omitting newer higher-dose versions of Novo’s drugs. It alleges violations of false-advertising and unfair-competition laws and seeks to stop the advertisements; Lilly denies wrongdoing and says its claims are supported by a direct head-to-head clinical trial.