Phentermine Detection In Urine

Phentermine, and amphetamine like drug, is generally detectable in the urine for at least 2 to 3 days.

Question

I was taking phentermine for two weeks. Will it show in my urine analysis this coming Monday?

Asked by Nina On Dec 14, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Dec 15, 2018

Phentermine (Adipex-P) is a stimulant, chemically and pharmacologically related to amphetamines (e.g. Adderall, Ritalin). Even if it not being specifically looked for on a urine drug test, it can produce a positive reading for amphetamines, according to most studies on the matter.

A commonly referenced guide on urine drug screening, Urine Drug Screening: Practical Guide for Clinicians, specifically lists phentermine and other related stimulants as commonly causing 'false-positives' for amphetamine:

Unfortunately, other stimulants, anorexiants [e.g. phentermine], and chemically related compounds (e.g., pseudoephedrine), have been shown to produce false-positive results, making the amphetamine assay one of the most difficult tests to interpret.
Urine Drug Screening: Practical Guide for Clinicians

In regard to how long it will last, or be detectable in the urine, depends on a number of factors including:

  • Hydration status
  • Kidney function
  • Urinary pH
  • Elimination half-life

The elimination half-life of a drug is typically a good indicator of how long it is detectable (although not always). After around 5 to 6 half-lives, most drugs are no longer detectable on standard urinalysis tests.

Based on published pharmacokinetic data on phentermine, it is primarily eliminated by the kidneys and has a half-life between 19-24 hours. This would translate to phentermine being detectable for 4-6 days after your last dose.

This is right in line with how long studies report other amphetamine drugs being detectable (generally listed at 4-5 days).

It is important to note that phentermine elimination is increased the more acidic the urine is. Some studies report that the half-life can decrease to 7—8 hours under acidic urinary conditions.

If your urine has a lower pH (i.e. more acidic) than normal, phentermine may be eliminated in as quick as 2-3 days.

Overall, if the drug test you are being administered is looking for phentermine, and you took your last dose on a Friday, there is a good chance it will be detectable on Monday (~3 days).

If the test isn't looking for phentermine, it still may produce a false-positive for amphetamines, although how often this occurs isn't well documented. We simply have case studies saying that it is possible and does happen from time to time. 

Answer Summary

Phentermine is detectable in the urine, anywhere from 2 to 6 days. It may be eliminated from the body more quickly under acidic urinary conditions.

Phentermine Information

Phentermine is classified as an anorexiant stimulant, and is chemically similar to amphetamines. It is indicated for the short-term treatment of obesity, when used as an 'add-on' to behavior modifications such a diet and exercise.

It is typically dosed before breakfast or 1 to 2 hours after breakfast. Afternoon/evening administration should be avoided due to the risk of insomnia.

The onset of action is quick, between 15-30 minutes after taking a dose and has a duration of action ranging from 12 to 14 hours (for the 30 and 37.5 mg capsules/tablets).

The most commonly reported side effects of phentermine include:

  • Headache
  • Insomnia
  • Overstimulation
  • Restlessness
  • Tremor
  • Increased blood pressure

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