Merck is a global pharmaceutical company that discovers, develops, manufactures, and sells prescription medicines, vaccines, and animal health products. The Pharmaceutical segment (~89% of product sales) is dominated by Keytruda (pembrolizumab), an anti-PD-1 cancer immunotherapy approved for more than 40 indications across 18 tumor types. Keytruda generated approximately $31.7B in FY25, nearly half of total company sales. Beyond Keytruda, key products include Gardasil/Gardasil 9 (HPV vaccine, $5.2B in FY25, though pressured by weak China demand), Winrevair (a newly launched PAH therapy with a novel mechanism), Bridion and Prevymis (hospital acute care), and a declining diabetes franchise (Januvia/Janumet). The Animal Health segment (~11% of product sales) sells veterinary pharmaceuticals and vaccines for livestock and companion animals, anchored by Bravecto, a parasiticide for dogs and cats. Merck's business model is R&D-intensive: the company invests heavily upfront in drug discovery and clinical trials, then earns high-margin revenues during patent exclusivity windows. Keytruda's compound patent expires in late 2028, and biosimilar entry afterward is Merck's central strategic challenge. To manage this, Merck is converting patients to subcutaneous Keytruda (patent to 2043), diversifying into ADCs, precision oncology, and individualized neoantigen therapy, and expanding into cardiometabolic (Winrevair, enlicitide oral PCSK9 inhibitor), respiratory (Ohtuvayre for COPD), HIV, immunology, and ophthalmology. Business development remains a priority, with recent acquisitions including Verona Pharma and several licensing deals to augment the pipeline.