How Long Does Phentermine Last In Your Body? | HelloPharmacist

Answer

Thanks for reaching out to us! 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 in the body is reduced. We have illustrated this below:

While the half-life of a specific drug is very indicative of how long the drug is detectable, there are other variables that 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.​

I do want to emphasize that 5 to 6 days is how long phentermine stays in your system (and could potentially be detected on a drug test). The duration of action of the drug (i.e. how long you actually feel the effect after a dose) is much shorter (4 to 12 hours).

Below, we have included some general data​ regarding phentermine and how it is metabolized in the body.

Administration And Metabolism

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

Phentermine is primarily excreted by the kidneys, which is why it is detectable in the urine.

The elimination half-life ranges from 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.

After taking a dose by mouth, 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.

Final Words

Thanks again for your question and please reach out to us again in the future!