Sounds like you've built up a tolerance to
phentermine. I don't know why your doctor is scratching his head; it's pretty common. I alternate appetite suppressants to avoid building a tolerance, and don't take them every day. I will usually take phentermine for 2 days, then
Didrex or
Tenuate for a day or two, then stop for a day or two. Then start over. If you take long acting versions, like the 75mg Tenuate or the 37.5mg phentermine, after a couple of days I find that I have enough built up in my system to go the couple of days without. At the very worst, I may take a short acting one in the late morning/early afernoon on the 2nd day.
If you've already got a tolerance to phentermine, which is sounds like you do, you need to get completely off it for a few weeks at least. Hope this helps.