Patent Expiry & Generic Competition: Merck faces revenue erosion as key drug patents expire, allowing generic competitors to enter the market and reduce sales volumes and prices for its established products.

R&D Failure & Pipeline Risk: The lengthy, costly, and uncertain nature of pharmaceutical research and development means that significant investments may not yield successful new drugs, leading to a lack of future growth drivers and pipeline gaps.

Regulatory Hurdles & Approval Delays: Obtaining and maintaining regulatory approvals for new and existing drugs is a complex process, with potential for delays, additional requirements, or outright rejections from health authorities like the FDA, impacting market entry and revenue generation.