I used phentermine, bought from a private diet clinic, about twenty years ago. They didn't help me because I got used to them so fast and had to keep upping the dose. Pretty soon they didn't work at all.
They stopped me sleeping. I was jittery and shaky a lot of the time. Worse still when I stopped taking them I experienced very strong 'rebound' hunger. This is common with amphetamine-related drugs.
My advice would be, don't take them. But of course it's up to you.
All the best!