Pharmaceutical King changes hands! Mounjaro from Lilly (LLY.US) surpasses Keytruda from Merck & Co., Inc. (MRK.US) to become the best-selling drug worldwide.

The heavyweight diabetes drug of Lilly (LLY.US) Mounjaro has surpassed Merck & Co., Inc.'s (MRK.US) cancer treatment drug Keytruda, becoming the best-selling drug worldwide. In the first quarter of 2026, Lilly's Mounjaro sales reached $8.7 billion, exceeding Merck & Co., Inc.'s Keytruda sales of $7.9 billion. Keytruda has been the best-selling drug globally since the first quarter of 2023, when it replaced AbbVie's (ABBV.US) autoimmune disease drug Humira. Mounjaro replaces Keytruda as the best-selling drug If Lilly's Mounjaro is combined with its weight-loss drug Zepbound for performance statistics, the performance is even more impressive. Both drugs use the same active ingredient tirzepatide. In 2025, the combined sales of these two drugs reached $36.5 billion, surpassing Keytruda's sales of $31.6 billion that year. Evan Seigerman, Managing Director of BMO Capital Markets, said, "The pharmaceutical market is moving from the era of Keytruda to the era of Tirzepatide, and this is the trend." He also pointed out that tirzepatide drugs are significantly effective and safe. "In my opinion, this shift is not surprising." He stated that the cancer drug market is completely different from the market for obesity and diabetes treatment drugs. When Keytruda was approved in 2014, it was revolutionary and extended the lives of patients who had previously been given a death sentence. Its pricing also reflected this. He stated that at the same time, tirzepatide provides an economically affordable treatment option for millions of obese patients whose condition does not pose a significant threat to life but greatly impacts quality of life. Analysts have been predicting for years that Lilly's drugs would become one of the best-selling drugs of all time, even though these drugs were launched later than Novo Nordisk A/S Sponsored ADR Class B's (NVO.US) Ozempic and Wegovy. Despite a large number of generic drugs flooding the market during drug shortages, and the Trump administration implementing policies to lower drug prices, Lilly's sales continue to soar. Meanwhile, Merck & Co., Inc. is facing a crisis as the patent for its immune therapy drug Keytruda is set to expire. Keytruda is used to treat various cancers, and its patent will expire in 2028, prompting the company to strengthen its research pipeline in other areas. However, obesity has not been a focus area for its research efforts so far.