What is the most effective weight loss pill for losing at least fifty pounds or more, using either OTC or prescription?

None. Studies show that probably no more than 8-10% of body weight will be achieved with weight loss pills. Unfortunately, no one can really stay on diet pills the rest of their life so weight regain is pretty much a given unless coupled with diet & exercise. There is no quick fix to losing weight other than hard work and good eating habits. Trust me, if there were a quick fix everybody would be thin.