Phentermine is the oldest and most widely prescribed weight loss medication in history — FDA-approved since 1959 and used by hundreds of millions of patients over six decades. It is a sympathomimetic stimulant that works by releasing norepinephrine in the hypothalamus, producing a powerful suppression of hunger signals that makes it the most immediate-acting of all available weight loss medications.
Phentermine is typically the entry point for medical weight management due to its low cost, proven efficacy for short-term weight loss, and extensive safety record. It is also a component of Qsymia (combined with topiramate) for long-term use. As a standalone medication, phentermine is approved for short-term use of up to 12 weeks, though it is commonly used for longer periods under medical supervision.
In clinical studies, phentermine produces an average weight loss of 5–7% of body weight over 12 weeks — less than GLP-1 medications but with rapid onset. Many patients notice significant appetite reduction within the first 24–48 hours of starting. It is best suited as an entry-level medication, a bridge while waiting for GLP-1 medications to take effect, or in combination with other agents for maximum impact.
