Novo Nordisk sued competitor Eli Lilly on Tuesday, alleging the drugmaker engaged in false advertising by claiming its GLP-1 drugs are superior to Novo's.

The lawsuit accuses Eli Lilly of misleading consumers by using outdated clinical trial data to make Zepbound and Mounjaro appear more effective than Novo's drugs, Wegovy and Ozempic, for weight loss and Type 2 diabetes treatment.

"In both cases, the ads are maliciously and deceptively false because Lilly knowingly cites outdated clinical trials that compare the highest doses of the Lilly medicines to lower doses of Novo Nordisk's medicines," according to the lawsuit, which was filed in the U.S. District Court in New Jersey.

In a statement to CBS News, Eli Lilly said it stands behind its advertising. "It is truthful, it is transparent, and it is grounded in the most direct scientific evidence available — exactly what patients deserve," the company said. "We will continue to focus on the science and defend against this lawsuit vigorously."

Novo Nordisk said the Food and Drug Administration has approved newer, more effective doses of its medicines that help customers achieve greater weight loss and blood sugar regulation. However, Lilly did not base the claims in its ads on the more recent dosage information, the lawsuit claims.

The Danish drugmaker said Lilly acknowledges the existence of a higher Wegovy dose in small footnotes in its ads, but claims they are "ambiguous" and "virtually invisible," according to the lawsuit.

The lawsuit comes as the two companies compete for dominance in the growing market for weight loss drugs. GLP-1 drugs have exploded in popularity in recent years, with 15% of U.S. adults reporting they've used the medicine for weight loss at some point, up from 12% last year, according to a June Gallup study.

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