14.02.3 Phentermine

Phentermine releases NA from nerves, (Figure 14.6), with a lesser ability to release DA and 5-HT.

Figure 14.6 Mechanism of action of phentermine (Copyright QUT, Sheila Doggrell)

Thus, phentermine is anorexic without causing a major central stimulation. Presently, this drug is available for short-term use in Australia: there use is limited to a maximum 12 weeks. This is primarily to prevent people becoming dependent on the remaining central stimulation effect. Clinical trials show that phentermine can cause about a 8% weight loss over a year.

The ability of phentermine to release NA is not limited to the CNS; this drug also release NA from sympathetic nervous system (SNS). This release of NA from the SNS underlies the contraindications to use of these agents. Thus, phentermine is contraindicated in cardiovascular disease especially hypertension, hyperthyroidism, and cerebrovascular disease.

The adverse effects of phentermine are due to the remaining small amount of central stimulation. Thus, common adverse effects with phentermine and diethylpropion include insomnia, restlessness and nervousness.