How Long Does Phentermine Stay in Your System?


Phentermine, a drug classified as an anorectic, is commonly prescribed as an appetite suppressant for obese patients. According to the U.S. Drug Enforcement Administration, Phentermine is the most widely prescribed and most frequently encountered diet pill on the market today. Phentermine, also known as brand names Adipex and Fastin, was manufactured to replace amphetamines, and should be used with caution.



In a person with average body build and metabolic rate, Phentermine can take from three to five days to pass through the body’s digestive system. This process ultimately depends on the height and weight of the patient. Those on the lower end of the scale can expect Phentermine to be eliminated from the bloodstream in as little as 72 hours.

Patients should be advised that if they are submitting a specimen for drug testing, Phentermine can in the body can result in a positive indication for amphetamines.



Phentermine can have varied results on a patient depending upon the amount of caffeine in the body. Caffeine has been shown to decrease the amount of Phentermine in the blood stream, thus weakening the effect of the medication. Caffeine pushes the drug through the body at a more rapid pace, moving it out of the system faster than the medicine alone.


Fluctuations in how long Phentermine stays in the blood stream can be induced by other medications. Prior to prescribing Phentermine, a physician will gather a medical history from the patient describing any previous or current drugs being taken.

Prescriptions such as anti-depressants, especially SSRIs, should be used with caution while taking Phentermine, according to The Doctors Lounge; since a dangerous condition called Serotonin Syndrome can occur. Medications, even herbal remedies and vitamins, can have an effect on the life of Phentermine in the body.


Body type and metabolic rate are two circumstances that could change the period that Phentermine stays in one’s bloodstream. As defined by, metabolic rate is the rate of metabolism, or the amount of energy used in a certain period.

If a patient has a thin body type and a fast metabolic rate where items are moved faster through the digestive system, then the length of time Phentermine stays in the system would decrease.



The usage of the drug is yet another factor in discovering the length at which it stays in the system. Phentermine is available in different strengths ranging from 15 mg to 37.5 mg, and various forms such as capsules and tablets. Some types of Phentermine are immediate release, while others are time-released, which ultimately effects the life in the bloodstream.

Phentermine is typically prescribed to be taken once per day, and it is cautioned that this drug can be addictive if misused. Those who abuse this medication and take more than prescribed by their physician, will obviously see a longer life of Phentermine in their system.