Phentermine is associated with modest weight loss in some people, but not in everyone. And as with all weight loss medications, once a person discontinues the medication, the weight tends to come back.
Everyone is different and things that may work for someone may not work for others. Make sure you are still getting healthy fats and other nutrients in your diet. I recommend trying something else or avoiding these weight loss products in general. Many of these products only work short term and could be unhealthy for you. The best way to lose weight is to eat healthy and play sports that you enjoy.