Mounjaro Becomes World's Best-Selling Drug in 2026

Mounjaro has officially overtaken Merck’s Keytruda to become the world’s best-selling drug, marking a major shift in the global pharmaceutical market.

The blockbuster diabetes treatment from Eli Lilly & Co. generated USD 8.7 billion in sales during the first quarter of 2026, surpassing the USD 7.9 billion posted by Keytruda, the cancer therapy developed by Merck & Co..

Keytruda had held the top spot since early 2023, when it replaced AbbVie’s Humira as the world’s highest-selling drug. But Lilly’s rapid rise in the obesity and diabetes treatment market has changed the rankings faster than many expected.

Lilly’s dominance becomes even more clear when combining sales of Mounjaro and its weight-loss counterpart Zepbound, both powered by the active ingredient tirzepatide. Together, the two drugs generated USD 36.5 billion in 2025, well ahead of Keytruda’s annual sales of USD 31.6 billion.

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“It makes sense that we’re shifting from King Keytruda to King Tirzepatide,” said Evan Seigerman, managing director at BMO Capital Markets Corp, noting how well the medicines work and how safe they are. “It’s not a surprise to me.”

Seigerman explained that cancer drugs and obesity treatment operate in very different markets. When Keytruda launched in 2014, it transformed cancer care by extending survival for patients with limited treatment options, which justified its premium pricing.

Meanwhile, tirzepatide-based treatments target millions of people dealing with obesity and diabetes, creating a much broader customer base. Despite copycat drugs during shortages and pricing pressure tied to the Trump administration’s affordability push, demand for Mounjaro and related GLP-1 medicines has continued to surge.

At the same time, Merck faces growing pressure as Keytruda’s patent expiration approaches in 2028, pushing the company to strengthen its future drug pipeline.