Long-term Phentermine Pharmacotherapy: An Investigation for Symptoms of Dependence, Cravings, or Withdrawal

Phentermine, an amphetamine congener, is the most widely used anti-obesity drug in the U.S. Although phentermine is the agent-of-choice among physicians specializing in obesity treatment, the use of this drug for obesity treatment by other physicians has long been curtailed because misapprehensions regarding phentermine safety. Concerns of phentermine-induced adverse cardiovascular reactions and of phentermine-induced addiction are two fears that have had a profound negative impact on phentermine prescribing. Although warnings of high incidence rates of adverse cardiovascular and psychiatric effects are included in FDA labeling and are often repeated in published reviews, the few clinical reports in the peer-reviewed medical literature of such adverse effects are anecdotal. Fear of phentermine adverse effects does not inhibit the use of phentermine by obesity treatment specialists. A 2008 survey of prescribing practices found that 98% of bariatric medicine specialists used pharmacotherapy in treating obesity and that 97% of those prescribed phentermine as their first choice.

The fear that phentermine has addiction potential appears to be a factor influencing curtailment of use. At the time that phentermine was approved in 1959 the expectations were that it would prove to be addicting, although perhaps less so than amphetamine. These expectations were based on the chemical structural similarities between phentermine and amphetamine and on evidence in rats that phentermine stimulated spontaneous activity. No evidence suggesting the drug had human addiction potential appeared in clinical trials conducted prior to approval.

After 52 years of use there is no evidence in the peer-reviewed medical literature to support the hypothesis that phentermine has significant human addiction potential. Research in addiction medicine has undergone significant development in the last 50 years. Concepts of addiction have shifted from an early focus on tolerance and withdrawal to a current emphasis on the psychological components of dependence. Drug addiction has been redefined as drug dependence and standardized diagnostic criteria have been adopted for drug abuse, dependence and withdrawal. Psychometric testing methods have been developed, validated, and applied clinically for measurements of dependence, drug craving, and withdrawal for a wide variety of substances of abuse including cocaine, heroin, and amphetamine.

Until recently, none of these addiction medicine metrics had been used to study the addiction potential of phentermine. Presumably, since phentermine is an amphetamine congener, any clinical characteristics of dependence or withdrawal should mimic those of amphetamine dependence or withdrawal. One recent retrospective study investigated symptoms occurring when patients treated with long-term phentermine in a weight management program abruptly ceased taking phentermine. The study found that patients on long-term phentermine who ceased phentermine abruptly by their choice did not have an amphetamine-like withdrawal symptom complex. Significantly there was no evidence of phentermine cravings. Further investigation is warranted.

The addiction potential of a drug may be investigated by measuring the drug's propensity to induce dependence, to induce cravings for the drug, and for cessation of the drug to induce characteristic withdrawal symptoms. In the case of amphetamine withdrawal symptoms appear very quickly reaching a maximum at 48 hours after drug cessation.

In this prospective study the addiction potential of phentermine will be assessed with validated psychometric scales to examine patients who have taken phentermine long-term for two years or more. Patients who have taken phentermine for 7 to 14 days will also be assessed. Participating patients who have taken phentermine long-term in this study will be asked to interrupt phentermine therapy for 48 hours to participate in the study. Scale examinations will be conducted at 24 and at 48 hours after drug cessation.

Hypotheses

1. Long-term phentermine-treated (LPT) patients do not develop phentermine dependence or cravings.
2. LPT patients who cease taking phentermine abruptly do not experience amphetamine-like withdrawal symptoms.

Specific Aims

1. To compare the severity of phentermine dependence and craving between LPT patients and acute phentermine-treated (APT) patients
2. To compare the severity of stimulant withdrawal symptoms before and after phentermine cessation in LPT patients.
3. To examine the prevalence of phentermine dependence in LPT patients