Amgen shares are selling off in extended trading following a regular session that saw the stock trading higher alongside a generally positive performance from its pharmaceutical peers. While recent reports highlighted competitive gains for the company, including the successful launch of a biosimilar to capture market share from Johnson and Johnson's Stelara, the stock is now facing downward pressure. Investors appear to be reacting to a post-close development, as the move follows recent analyst commentary that noted a neutral Hold rating due to concerns over declining sales from older products like Prolia and Xgeva following their 2025 patent expirations.