It can cause insomnia and should be avoided in the evening or at night. Phentermine can cause increased blood pressure, palpitations and arrhythmias. It is contraindicated in patients with high blood pressure and in patients with underlying heart abnormalities.

Phentermine is structurally similar to amphetamine, but it is less harmful to the central nervous system and less addictive. Its mechanism of action is mainly related to its enhancement of noradrenergic (NE) and dopamine neurotransmission, which in turn produces an appetite suppressant effect.

Phentermine is available in immediate-release and extended-release dosage forms.

However, with a reported phentermine plasma half-life of 12 - 24 hours, the value of extended-release formulations is questionable. Phentermine is an effective adjunctive medication that, in combination with dietary control, exercise, and behavioral modification produces significantly greater weight loss than placebo.

Intermittent phentermine therapy appears to induce considerable weight loss compared with continuous use. However, most people usually regain it after discontinuing use. A single dose of 30 milligrams once a day in the morning can be effective in suppressing appetite throughout the day. However, doses of 8 mg immediately before meals are common. A daily dose of more than 30 milligrams does not improve efficacy.