*4.1.1 Phentermine*

*Phentermine* is a sympathomimetic amine approved in 1959. It increases hypothalamic catecholamine secretion and resting energy expenditure to suppresses appetite. Phentermine is the most frequently used obesity medication in the USA [96]. It is indicated for short-term (3 months), as no long-term safety research exists, but it was approved in combination with topiramate extended release (ER) for long-term therapy. However, many obesity physicians prescribe it for >3 months as off-label therapy for weight management [96]. It is available in 8, 15, 30 and 37.5 mg (single dose early in the day to prevent insomnia). Side effects are mild and due to sympathomimetic effects e.g., dry mouth, insomnia, agitation, constipation and tachycardia. Administering the lowest effective dose decreases the side effects. Phentermine is a schedule IV-controlled substance, and it is contraindicated in patients with history of cardiovascular disease, anxiety, hyperthyroidism, drug or alcohol abuse/dependence. Other contraindications include concomitant treatment with monoamine oxidase inhibitors, pregnancy and breastfeeding [97]. Expected WL with phentermine is 3.6 kg at 6 months [98]. After 6 months, phentermine 15 mg induced a WL of 4.5 kg over placebo [99]. Forty-six percent of participants on phentermine lost ≥5% of their weight, while 20.8% lost ≥10% [99].
