Is Phentermine sold online the same as the prescription Phentermine?

2 years ago I was prescribed Phentermine by my doctor. I am looking to get on it again and have found many websites that actually sell it. I thought it was by priscription only? Does anyone know what the difference is?
The stuff I had prescribed was only 15mg and they have some advertised with as much as 30mg. I just dont want to take something that could potentially kill me?!