There are no official clinical studies on how long tolerance to phentermine will last. Patient reports indicate that it may last for months or years after you stop taking phentermine. Although phentermine is not “addictive”, it produces tolerance over time. Tolerance can be good or bad and occurs due to the body's attempt to create homeostasis (or a predictable balance).
In the case of phentermine, reducing thirst and insomnia may be beneficial; however, loss of appetite, energy suppression is less desired. Plus, it's not phentermine that stopped working; it's the fact that your body is intelligent. Many people don't realize that tolerance to phentermine is a possibility. It learns the patterns of our behavior and defends us.
After several rounds of weight gain and loss, with or without phentermine, it's very likely that you can no longer control your weight loss so easily. You'll need to change your habits. Tolerance to phentermine is an especially common problem in patients who have been taking phentermine for a long time or who are taking it for the second or third time. In an effort to make phentermine work better, many patients seek supplements to take along with phentermine.
Phentermine is a popular and effective weight-loss medication, but unfortunately, many users report that phentermine stops working after 2 to 3 months of use. If you expect phentermine to shed those extra pounds like a magic trick, you should abandon phentermine treatment or change the way you think about weight loss.