One of the pharma giant's top-selling products is about to face some intense competition.It's no secret why Pfizer (PFE -0.82%) stock has been struggling this year. Investors are worried about how well the company will be able to do in light of waning demand for its COVID-19 vaccine and pill. It's also facing patent cliffs for many of its top drugs. That's why even though it's trading at a seemingly cheap 12 times its estimated future earnings, investors aren't piling into the stock. And now, one of its riv ...