The news: Novo Nordisk filed a lawsuit against Eli Lilly in federal court in New Jersey, alleging the company's national ad campaigns for Zepbound and Mounjaro use outdated clinical data to falsely suggest the drugs outperform Novo's Wegovy and Ozempic. The complaint cites violations of federal and state false advertising and unfair competition laws, including under the Lanham Act.

Novo is seeking a permanent injunction requiring Lilly to pull the ads, a corrective ad campaign, and financial damages, including any profits Lilly earned from the campaigns. Novo also plans to seek a preliminary injunction in the coming days to immediately block the ads.

Zooming in: Novo's complaint targets weight loss and diabetes drug ad comparisons it says mislead consumers by pitting Lilly's highest doses against Novo's lower, outdated ones.

• Weight loss drug claims: Novo claims Lilly's Zepbound campaign compares its 10 mg and 15 mg doses with Wegovy's 1.7 mg and 2.4 mg doses while omitting Wegovy's highest FDA-approved 7.2 mg dose, which was approved in March 2026 and produced average weight loss of 19% (about 47 pounds), according to Novo. This would be clinically consistent with the 50-pound weight loss featured in Lilly's Zepbound ad.
• Diabetes drug claims: Novo says Lilly’s Mounjaro campaign compares the drug’s highest 15 mg dose with Ozempic’s 1 mg dose, omitting the FDA-approved 2 mg maintenance dose that has been available for more than four years.
• There is no published head-to-head study comparing the current highest doses of Zepbound and Wegovy, Novo stated.

The Zepbound campaign has run at national scale. It has aired during major sporting broadcasts and on TikTok and Facebook, with the TV ad drawing more than 700 million impressions since late April, per CNBC.

Novo sent Lilly a cease-and-desist letter in April and says it never received a response. Lilly's ad does include a footnote referencing the 7.2 mg Wegovy dose, which Novo calls inadequate and hard to read.

Why it matters: The lawsuit comes as Lilly leads in most major GLP-1 metrics, including prescription share, sales growth, and market value. On its latest earnings call, Lilly said it holds roughly 60% of the US GLP-1 market, versus Novo's 39%.

Novo has seen early momentum with oral Wegovy compared with Lilly's oral Foundayo, but it still has significant ground to make up: Lilly's GLP-1 portfolio has delivered much faster sales growth over the past year. Lilly is the world’s most valuable pharma company with a market cap surpassing $1 trillion, with shares up nearly 50% over the past year, while Novo's stock has fallen about 25%.

Novo's decision to sue Lilly underscores the high stakes in a GLP-1 market projected to reach $200 billion by the end of the decade. In a market this large, even modest competitive gains with physicians, consumers, investors, and Wall Street can shift the balance of power. For instance, many consumers ask physicians about GLP-1 ads they see on TV and online, and perceived differences in weight loss efficacy can heavily influence prescribing decisions.

Implications for pharma companies and marketers: The most likely near-term outcome is that Lilly will revise its ad campaigns, whether voluntarily or court-ordered. The lawsuit reasserts the clinical case for Wegovy while Novo’s GLP-1 injections lose prescription share, and could force Lilly to spend time and money defending its ad claims.

Pharma marketers should watch how comparative claims hold up once multiple approved doses exist for competing drugs. As newer doses reach the market, campaigns built on older trials become vulnerable to challenge. Legal teams should ensure comparative safety and efficacy claims are supported by scientifically appropriate, statistically sound evidence, which is typically from head-to-head clinical trials rather than comparisons across separate, independent studies.