Phentermine was FDA-approved for weight loss in 1959, and it is by far the most widely prescribed diet drug ever. Yet, many providers are apprehensive to prescribe it. While no medication is completely benign, there are a number of misconceptions about phentermine and its risks. To help demystify the drug, we've taken some common beliefs, examined the evidence, and broken everything down for you below.

• Phentermine is a controlled substance and can be abused - While phentermine is an atypical amphetamine analog and schedule IV controlled substance, it differs from other commonly abused stimulants. The addictive properties of cocaine and ADHD drugs like Adderall center around their effects on dopamine release. Phentermine mainly increases norepinephrine activity and has minimal effects on dopamine. Studies have shown that even after 21 years of use, patients develop little to no psychological or physical dependence on it. Perhaps most telling is the fact that drug abusers don't seek it, and it has no street value.^1,2
• Phentermine can cause heart valve disease and/or pulmonary hypertension - In the 1990s, phentermine was combined with fenfluramine in the popular "Phen-fen" diet pill. Fenfluramine and another drug called dexfenfluramine, both serotonergic agents, were later found to cause valvular heart disease and pulmonary hypertension; these effects are believed to occur through direct stimulation of serotonin receptors (5-HT2b) in the heart. In 1997, the FDA pulled fenfluramine and dexfenfluramine from the market and, at the same time, placed a warning about these events on phentermine. No evidence prior to or after that time has linked phentermine to these conditions. Furthermore, studies that have looked specifically for an association have found none.^1,3
• Phentermine can raise blood pressure - since phentermine increases catecholamine (e.g. norepinephrine, epinephrine) activity, it theoretically can raise blood pressure. However, studies have shown that average blood pressure typically declines in phentermine-treated patients, including those with hypertension, with a significant drop seen as early as one week after initiation. Reasons for this effect may include weight loss and/or suppression of sympathetic activity by norepinephrine in the CNS.^1,4
• Phentermine can only be taken for short periods - the original prescribing information for phentermine recommends that it be given for a maximum of 3 months. In practice, many patients take it off and on or continuously for years. Phentermine is also a part of Qsymia (phentermine + topiramate), which has been studied for up to a year and has no recommended limit on duration. Incidentally, weight loss with Qsymia is no better than phentermine monotherapy in trials (∼10%).^5,6
• Phentermine is ineffective - studies show that phentermine causes average weight loss of 10%, which is second only to Wegovy (16%) among available obesity treatments. Over time, patients can develop tolerance and weight loss can plateau, but this is true of any weight loss therapy.⁶

Phentermine, FDA-approved since 1959, is effective, safe, and one of the most widely prescribed drugs ever. With an average weight loss of 10% and a cost of $10 a month, it is by far the most cost-effective diet drug available. It's perplexing that some providers are afraid to prescribe it yet have no issues giving someone Adderall for life.

• Phentermine patient handout

¹ PMID 24621808
² PMID 23736363
³ PMID 9731087
⁴ PMID 21527891
⁵ PMID 21481449
⁶ PMID 20920040