Eli Lilly's (LLY.US) blockbuster diabetes drug Mounjaro has surpassed Merck's (MRK.US) cancer treatment Keytruda to become the world's best-selling medication. In the first quarter of 2026, sales of Lilly's Mounjaro reached $8.7 billion, exceeding the $7.9 billion in sales for Merck's Keytruda. Keytruda had been the global top-selling drug since the first quarter of 2023, when it replaced AbbVie's (ABBV.US) autoimmune disease drug Humira.
The performance is even more impressive when Mounjaro's sales are combined with those of Lilly's weight-loss drug Zepbound, as both drugs share the same active ingredient, Tirzepatide. In 2025, the combined sales of these two drugs totaled $36.5 billion, surpassing Keytruda's sales of $31.6 billion for that year.
Evan Seigerman, Managing Director at BMO Capital Markets, stated, "The transition in the pharmaceutical market from the Keytruda era to the Tirzepatide era is an inevitable trend." He noted that Tirzepatide-based drugs demonstrate significant efficacy and a high safety profile. "In my view, this shift in the landscape is not surprising," he said, pointing out that the cancer drug market is fundamentally different from the markets for obesity and diabetes treatments.
When Keytruda was approved in 2014, it was revolutionary, extending the lives of patients who previously had limited options. Its pricing reflected this breakthrough. In contrast, Tirzepatide offers an affordable treatment option for millions of patients suffering from obesity—a condition that, while not fatal, significantly impacts quality of life.
Analysts have long predicted that Lilly's drugs would become among the best-selling medications ever, even though they entered the market later than Novo Nordisk's (NVO.US) Ozempic and Wegovy. Lilly's sales have continued to surge despite generic competition during drug shortages and ongoing pricing pressure on the GLP-1 receptor agonist market from policies like the Trump administration's drug price reduction efforts.
Meanwhile, Merck faces the challenge of Keytruda's patent expiration. Keytruda, used to treat various cancers, will lose patent protection in 2028, prompting the company to strengthen its research pipeline in other areas. So far, however, obesity has not become a major focus of its development efforts.
