Phentermine usually stays in your system for several days, and it can be detectable on some drug tests for up to a week or more, depending on the type of test and your body’s metabolism.

How Long Does Phentermine Stay in Your System?

Phentermine is a stimulant medication used short term for weight loss, and your body clears it gradually over a few days. The exact timeframe depends on dose, how long you’ve been taking it, your age, kidney function, and how fast your metabolism works.

Typical elimination time

Most medical sources point to:

  • Half-life: About 19–24 hours (time for half the drug level to drop).
  • Total clearance from body: Around 4–6 days for most people.

That “4–6 days” comes from the rule that it takes roughly five half-lives for a drug to be effectively eliminated.

How Long It Shows on Drug Tests

Different tests detect phentermine for different lengths of time.

  • Urine tests: Commonly detect phentermine for about 2–5 days after your last dose; in some cases up to about a week.
  • Blood tests: Usually detect it for roughly 24–72 hours after the last dose.
  • Saliva tests: Similar to blood, generally up to a couple of days (exact data is more limited, but often quoted in the 1–2 day range).
  • Hair tests: Can potentially show use for up to about 90 days, as with many other stimulants.

If you take phentermine exactly as prescribed and are facing a drug screen, it’s important to disclose that prescription in advance so results are interpreted correctly.

What Affects How Long It Stays?

Several personal and treatment factors change how long phentermine remains in your system.

Key factors include:

  • Dose and frequency: Higher doses or longer use can mean slightly longer detection times.
  • Metabolism: People with faster metabolisms clear the drug more quickly; a slower metabolism extends the window.
  • Kidney function: Phentermine is mainly eliminated by the kidneys, so reduced kidney function can prolong its presence.
  • Body weight and fat: Body composition can subtly affect how long stimulants linger.
  • Route of use: Using it as prescribed by mouth leads to the usual clearance times; non‑prescribed routes (like snorting or injecting) may change both effects and how long it’s detectable and also increase health risks.

An example: a healthy adult taking a standard oral dose once daily for a few weeks will typically stop having detectable levels in urine within several days of the last pill, though a hair test might still show use for months.

Quick Safety Notes

  • Do not start, stop, or change your phentermine dose without talking to a healthcare professional; suddenly stopping after long use can sometimes cause rebound appetite and other symptoms.
  • If you’re worried about an upcoming test or side effects (like racing heart, high blood pressure, insomnia, or mood changes), your prescriber or pharmacist can give personalized advice.
  • If you’re using phentermine without a prescription or in higher doses than prescribed and are finding it hard to cut back, that’s a medical issue worth discussing with a clinician or an addiction specialist.

Information gathered from public forums or data available on the internet and portrayed here.

TL;DR: Phentermine’s half-life is about 19–24 hours, and for most people it is largely cleared in about 4–6 days, with urine tests typically positive for around 2–5 days and hair tests for up to 90 days.