Substance guides
How long does phentermine stay in your system?
Published March 12, 2025 · 6 min read · Updated April 2026
Reviewed for accuracy by licensed clinical professionals.
Phentermine is a prescription weight loss medication that is structurally similar to amphetamine. It has a half-life of approximately 20 hours.
Detection
Urine: 3-7 days. Important: phentermine can cause a positive result on standard amphetamine immunoassay screens. Confirmatory GC/MS testing distinguishes phentermine from amphetamine. Blood: 1-2 days.
The false positive issue
Because phentermine is structurally similar to amphetamine, initial drug screens may report a positive amphetamine result. A valid prescription and confirmatory testing resolves this.
What to do
If you take phentermine and face a drug test, inform the Medical Review Officer of your prescription before the test.
Frequently asked questions
Does phentermine show as amphetamine on a drug test?
Initial immunoassay screens may show positive for amphetamines. Confirmatory testing distinguishes phentermine from amphetamine.
How long does phentermine stay in urine?
3-7 days due to its relatively long half-life of 20 hours.
Is phentermine addictive?
Phentermine has low abuse potential compared to other amphetamine-type stimulants, but dependence can develop with chronic use.
Disclaimer: Informational only. Not medical advice. SAMHSA: 1-800-662-4357.