How Long Does Phentermine Last In Your Body?

One of our most common questions at PharmacistAnswers is regarding how long a certain drug stays in the body for. There very often isn't a clear cut answer but we certainly can make a good estimate based on what we know about drug metabolism as well as the specifics of the drug in question. See below for our latest question and answer regarding the weight loss drug phentermine.


How long does phentermine stay in your system? When will it stop showing up in a urine test?

Asked by Phoebe On Sep 28, 2017

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Sep 28, 2017

While you don't mention in your question what strength of phentermine you took, it doesn't influence the answer much, if at all. The part of the drug profile we are going to look at to determine how long it stays in your system is 'half-life'. 'Half-life' simply refers to the time it takes the body to metabolize 50% of the drug. In other words, for every half-life, 50% of the drug remaining the body is reduced. We have illustrated this below:

Half Life Drug Metabolism

While the half life of a specific drug is very indicative of how long the drug is detectable, there are other variables can influence the amount of time a drug remains detectable including the type of test being used, whether or not the drug has detectable metabolites, length of time using the drug etc... So keep in mind that half life is but one factor.

For phentermine specifically, it has been mostly determined that the drug is detectable on a standard urinalysis test for about 5-6 half lives.The half-life of phentermine in the body is around 19-24 hours.So theoretically, phentermine will be detectable for about 5-6 days, based on the test that is being used.​>Below, we have included some general data​ regarding phentermine and how it is metabolized in the body:


Phentermine is administered by mouth. There are no injectable formulations of the drug. It does not matter if the drug is given with food or an empty stomach as that does not influence the properties of the drug.


Phentermine is primarily excreted by the kidneys. The elimination half-life ranges 19—24 hours as mentioned above in the article and slightly is influenced by urinary pH. An acidic environment lends the half-life to decrease to about 7—8 hours. Following administration, most absorption of phentermine occurs from the small intestine. The duration of action following administration of the 8 mg capsules or tablets is about 4 hours and 12—14 hours after administration of the 30 mg capsules or the 37.5 mg tablets.

About the Pharmacist

Dr. Brian Staiger Pharm.D

Dr. Brian Staiger is a licensed pharmacist in New York State and the founder of He graduated from the University At Buffalo with a Doctor of Pharmacy degree in 2010. He has been featured in numerous publications including the Huffington Post as well as a variety of health and pharmacy-related blogs. Please feel free to reach out to him directly if you have any inquiries or want to connect! He's answered thousands of medication and pharmacy-related questions and he's ready to answer yours! Office: 716-389-3076

Recent Questions