Phentermine generally stays in your system for about 3 to 7 days after the last dose, with the average elimination time around 4 to 6 days. This duration depends on individual factors such as metabolism, kidney function, urine pH, dosage, and frequency of use.
- The elimination half-life of phentermine is approximately 20 to 25 hours, meaning it takes about this long for half of the drug to be cleared from the body
- Because it takes about 5 to 6 half-lives for a drug to be fully eliminated, phentermine can remain detectable for roughly 4 to 6 days
- Acidic urine (pH < 5) can shorten the half-life to 7-8 hours, speeding elimination, while alkaline urine may slow it down
- Phentermine is primarily excreted unchanged in urine (62-85%), so kidney function significantly affects clearance
- Detection windows vary by test type: urine tests can detect phentermine for 3 to 5 days, blood tests up to 48 hours, and hair tests up to a month or longer
In summary, most people will have phentermine cleared from their system within about 4 to 6 days, but this can vary based on personal health and metabolic factors. The drug’s appetite-suppressant effects typically last 1 to 2 days after ingestion, even though the drug itself remains detectable longer
