Phentermine is an amphetamine that is prescribed to reduce appetite. It won't actually cause fat loss, but simply reduce appetite. Amphetamines will raise cortisol levels in the body and can lead to insomnia, head aches, adrenal fatigue etc.
Overall, it isn't a good idea. What comes up, must come down. Any prescription appetite suppressant is going to work but as soon as the drug wears off, you are going to be ravenous and binge.
Here are some natural tips to help cut appetite:
1. Hot drinks help immensely with hunger. Especially green tea.
2. Carbonated drinks add volume to your stomach and make you feel full. I drink perrier constantly when pre-contest dieting.
3. Stay busy. If you are constantly occupied you won't noticed how hungry you are.
4. Mindfully eat. Don't eat while on the computer or infront of the television.
5. Caffeine greatly cuts appetite -> be careful with this one. Too much caffeine will burn out your adrenal glands and raise cortisol levels
6. Supplement with 10g psyllium husk before bed. Psyllium husk is a soluble fibre and gels up in your stomach, leading to an increased feeling of satiety.