While the active ingredient 'phentermine' is exactly the same in both generic phentermine and branded phentermine, phentermine brands can contain different inactive ingredients. Inactive ingredients include coloring and coating to make the tablets easier to swallow, or ingredients which delay the release of active ingredients. Duromine, for example, a phentermine brand sold in Australia, New Zealand and South Africa, combines a slow-release resin with phentermine to delay the release of the phentermine effects of appetite suppression and energy.
Some phentermine brands differ in more than inactive ingredients; the phentermine brand Qsymia combines phentermine and the additional active ingredient, topiramate. Topiramate is an anti-convulsant drug which can cause weight loss as a side effect, and it has been found to strengthen the appetite suppressant effects of phentermine.
Generic phentermine is however, much cheaper than phentermine brands, which is something you find with all medications. This is because there is much less money spent advertising, promoting, marketing, researching and developing the generic version of a drug. Second, the profit margins of generic drugs like generic phentermine are considerably less than those of their brand name equivalents. Third, because there are usually several providers of a given generic (as with generic phentermine which is manufactured by several different pharmaceutical companies), competitive pricing often occurs among different generic versions of the same brand name product.
Some people may find that phentermine brands are more effective than generic phentermine, but the opposite can also be true. This is because your reaction to a particular medication is very dependent on individual differences in your physiology and lifestyle. So, just as you may experience different side effects and to different degrees when compared to other people, you may also experience differences in the desired effects of phentermine when comparing generic phentermine to phentermine brands.