Are there any major differences between phentermine and hoodia?

Yes. Phenteramine is a controlled prescription stimulant that does cut appetite and promote weight loss. However, most patients gain back the weight they lost quickly. Hoodia is an over the counter herb, also a stimulant, however there is little evidence it helps one lose weight. Also has un-wanted side effects.
Yes. Phentermine is used for a limited period of time to speed weight loss in overweight people who are exercising and eating a low-calorie diet. . Phentermine is in a class of medications called anorectics. It works by decreasing appetite. Goofus is a natural plant supplement with similar activity.