Is phentermine addicting and is long-term use safe?

Phentermine was approved for use in 1959 and for a period of 12 weeks. As obesity has been identified as a chronic disease, the long term use of phentermine has become mainstream amongst physicians who practice full-time obesity medicine. 97% of obesity medicine specialists indicate phentermine is their drug of choice among the current medication options available. Reasons for this include efficacy, safety, tolerability, low cost and low addiction potential. Despite this, concerns regarding safety and addiction potential have continued to limit the use by some physicians.

The Study: Hendricks et al sought to study the addiction potential of phentermine by assessing for craving and withdrawal upon cessation of phentermine. Study participants (N=117) had taken phentermine for at least one year. Control participants (N=152) took phentermine for a period of less than 2 weeks. The majority of treatment participants took doses between 18.75 and 37.5 mg daily, though some took doses much higher, and median treatment duration in the study group was 7.2 years.

Results: After cessation of phentermine, there were no signs of phentermine abuse, cravings or withdrawal. The only symptom noted after cessation was increased appetite. Notably, there were no symptoms usually associated with amphetamine withdrawal.

Discussion: Phentermine has been in use for over 50 years. There are no studies to date that indicate it has addiction potential, and this study confirms there is no withdrawal or craving when discontinued. While phentermine is FDA approved for short term use, the cost to do studies of long term use are cost prohibitive, since phentermine is a generic medication. Therefore, it is highly unlikely the FDA label will change. Of note, Qsymia (phentermine plus Topiramate) is FDA approved for long term use. Statements by The Endocrine Society and Obesity Medicine Associations support the off label use of phentermine for the long term management of obesity.

Addiction potential of phentermine prescribed during long-term treatment of obesity. International Journal of Obesity (2014) 38, 292–298; doi:10.1038/ijo.2013.74