One of the major drawbacks of using Phentermine to lose weight is that it can be habit forming in some patients. As a result, the patient may experience severe withdrawal symptoms when they stop taking the medication and these symptoms will need to be supervised by a physician. Due to the addictive nature of this drug, some people with a prior history of addiction with other substances are not advised to take Phentermine. Phentermine without prescription can be harmful to health. So consult your family doctor first before use.