Phentermine has been shown in clinical studies to assist patients in reaching their weight loss goals. The medication may be prescribed intermittently, meaning that you take a break from it for a set period before resuming it.

It is important to note, however, that not everyone will react the same way to this medication. After 3 months, a provider may recommend discontinuing it if you haven’t lost much weight. Its effectiveness appears to gradually decline after a year of continuous use.

In addition to its effectiveness for weight loss, Phentermine may be more effective when combined with topiramate. In addition to treating seizures, topiramate also reduces appetite, like Phentermine. The combined topiramate and phentermine was approved by the FDA in 2012 as an addition to healthy lifestyle changes including regular exercise and calorie restriction.

Phentermine and topiramate were found to have the highest odds of reducing body weight by at least 5% compared to three other commonly prescribed drugs. In addition, a recent analysis of studies found that people taking Phentermine and topiramate lost an average of 16 pounds compared with people taking a placebo. This number increased to 17.8 pounds for those who took the medication for more than 56 weeks. Additionally, the amount of weight lost increased with the dose.

Phentermine usually results in a weight loss of 3% of your initial body weight after 3 months and 5–7% after 6 months of use. Some people have lost even more weight than that. For a patient who weighs 200 pounds, a weight loss of 5–7% is 10–14 pounds.

Weight loss begins with a well-balanced, low-calorie diet and regular exercise, but some drugs can help support those efforts. Phentermine is one of the world’s most popular weight loss medications. Along with a healthy, calorie restricted diet and regular exercise, it has been shown to be effective for short-term weight loss.

A prescription is required to purchase phentermine, which is a stimulant. The risk of dependence associated with phentermine makes it a schedule IV controlled substance that is strictly regulated by the DEA. Chemically, it is similar to the controlled substance amphetamine.

Phentermine takes about three to five days to build up in the system and begin to decrease appetite. 

Phentermine also takes about three to five days to fully leave the body, so if a day is missed, there is still quite a bit left. In spite of the fact that forgetting a day here or there seems to have little effect, those who lose the most weight tend to be the most regular with their phentermine doses.