Since the 1950s, pharmaceutical companies have raced to discover and develop billion-dollar blockbuster drugs. In the face of stiff competition, only a handful are able to remain relevant over the course of many years; Rituxan, Advair and Humira are several that come to mind. But which of these can lay claim to being the best selling drug of all time? With millions of prescriptions every year since 1996, Pfizer’s anti-cholesterol drug atorvastatin (Lipitor) is still going strong over 20 years later.