Phentermine

The verdict

Our read at a glance.

Whether a drug labelled for a few weeks of use is safe taken for years, which is how it is actually used.

The signal

The efficacy evidence is old but solid: phentermine produces meaningful weight loss against placebo, and it remains the most widely prescribed anti-obesity medication historically, supported by labelled products and a continuing trial literature including combination work. The gap that defines this memo is between the label and the practice. It is approved as a short-term adjunct - a few weeks - because that is how the original trials were run, while real-world prescribing routinely extends for months or years. Retrospective cohort work examining longer-term use has been reassuring on cardiovascular outcomes relative to the fen-phen era, but that reassurance comes from observational data, not from trials designed to answer the question.

The unknown

Long-term cardiovascular safety on a proper trial footing. The drug's reputational damage came from fenfluramine, its former combination partner, not from phentermine itself - but nobody has run the long-duration randomized trial that would settle it.

Our read

A cheap, effective drug used in a way its label does not cover, on safety reassurance that is observational rather than trialled. It is also routinely blamed for harms caused by a different drug it was once paired with - which is worth getting right in both directions.