This list of top-selling drugs could look drastically different in a few years.You might think that if a drugmaker has a top-selling drug, it would be a lock to be a top-performing stock and a no-brainer buy. That may be true in some cases. In others, it can be a source of risk as investors grow concerned about what a company's growth opportunities will look like after the relevant patent expires, especially when the drug accounts for a significant chunk of the company's sales.The three top-selling drugs in ...