Question: how much weight can you lose in one week on phentermine?
Answer: Effective for Weight Loss The expected average weight loss with phentermine use is 5% of your initial body weight. Yet, over 12 weeks, it can be as high as 10%. This equates to a weight loss of 10–20 pounds (4.5–9 kg) for a 200 pound (90.7 kg) person ( 8 ).