Eli Lilly (LLY)’s flagship diabetes drug Mounjaro has surpassed Merck & Co.’s (MRK) cancer immunotherapy Keytruda to become the world’s best-selling drug. In the first quarter of 2026, Mounjaro achieved sales of $8.7 billion, higher than Keytruda’s $7.9 billion over the same period. Since replacing AbbVie (ABBV)’s Humira in the first quarter of 2023, Keytruda had long held the top spot in global drug sales, a position now overtaken by Mounjaro.
When combining Mounjaro with Eli Lilly’s other weight-loss drug, Zepbound, the performance is even more impressive. These two drugs share the same active ingredient, Tirzepatide, and together generated $36.5 billion in sales in 2025, also surpassing Keytruda’s $31.6 billion in sales for that year.
Evan Seigerman, Managing Director at BMO Capital Markets, stated that the shift from the Keytruda era to the Tirzepatide era in the pharmaceutical market is an inevitable trend. He pointed out that given the remarkable efficacy and high safety profile of Tirzepatide-class drugs, this shift in landscape is not surprising. He further analyzed that the cancer drug market is fundamentally different from the market for obesity and diabetes treatments. Keytruda, approved in 2014, was revolutionary, extending the lives of patients who previously faced death sentences, and its pricing reflected that value. In contrast, Tirzepatide offers a relatively accessible treatment option for millions of patients with obesity – a condition that is not immediately fatal but severely impacts quality of life.
Despite Novo Nordisk’s (NVO) drugs Ozempic and Wegovy launching earlier, and market pressures including the entry of generic drugs during shortages and price reduction pressures from the Trump administration’s drug pricing policies, Eli Lilly’s sales continue to surge. Meanwhile, Merck & Co. is facing the looming patent expiration of Keytruda in 2028 and has been strengthening its R&D pipeline in other areas, though obesity has not been a primary focus for the company to date.
Eli Lilly’s announced first-quarter results significantly exceeded expectations, leading the company to raise its fiscal year 2026 revenue guidance to $82–85 billion, with non-GAAP earnings per share expected to be between $35.50 and $37. The strong performance of Mounjaro and Zepbound is the primary driver, confirming that the company’s recent growth prospects are now built on a solid foundation of profitability.
The strategic focus is now shifting to Eli Lilly’s newly launched oral GLP-1 receptor agonist, Foundayo. If this drug succeeds, Eli Lilly will be able to expand its market reach to cover a broader patient population earlier, particularly those in early treatment stages or unwilling to accept injections, thereby solidifying its position in the increasingly competitive obesity market. Management’s upward revision of the financial outlook further confirms that Eli Lilly already possesses strong momentum before Foundayo delivers significant financial returns. The company’s core obesity and diabetes product portfolio continues to generate exceptional market demand, providing the flexibility to increase investments in manufacturing, commercialization, and future pipeline expansion.
