Phentermine

Phentermine is the oldest weight-loss drug still in common use in the United States — its label lists an initial U.S. approval of 1959. It is a sympathomimetic amine, chemically and pharmacologically related to the amphetamines, and it works by suppressing appetite through stimulant activity in the brain.

Read the indication carefully, because it is unusual. The label describes phentermine as a short-term (a few weeks) adjunct to a regimen of exercise, behavioral modification and caloric restriction, for patients with an initial BMI of 30 or higher, or 27 or higher with weight-related risk factors. “A few weeks” is the labeled duration. Long-term prescribing happens, but it is off-label, and patients are rarely told that.

The contraindication list reflects its stimulant nature: a history of cardiovascular disease including coronary artery disease, stroke, arrhythmias, congestive heart failure or uncontrolled hypertension; use during or within 14 days of a monoamine oxidase inhibitor; hyperthyroidism; glaucoma; agitated states; and a history of drug abuse. The label also notes that phentermine has not been studied at an eGFR below 15 and should be avoided there. It is a Schedule IV controlled substance, and the label warns about tolerance developing.

The confusion worth clearing up is the comparison people make with GLP-1 drugs. Phentermine is far cheaper and works quickly, but it is labeled for weeks rather than years, it carries cardiovascular contraindications that obesity itself often produces, and tolerance to its appetite effect is expected. It is a different tool for a different window, not a budget version of the same thing.

Worth asking your clinician: what the plan is when the labeled short course ends, whether your blood pressure and heart history rule it out, and whether the combination product Qsymia or a GLP-1 fits your situation better. Our phentermine versus Ozempic comparison and combination page cover the options.