How Long Does Adipex (Phentermine) Stay in Your System?

Adipex in Your Blood, Urine, & Hair

Table of Contents
View All
Table of Contents

Adipex (phentermine) is a prescription medication given to assist people with weight loss. It works by decreasing the appetite and is usually prescribed for three to six weeks (though can be used for up to 12 weeks) to people who are exercising and eating a low-calorie diet.

Phentermine is also the active ingredient in other prescription drugs including Fastin, Ionamin, and Zantryl and is one of the two ingredients in Qsymia. These medications are the most prescribed diet pills on the market. Because these drugs are similar to amphetamines and act as a stimulant, they are a controlled substance and are often sold illicitly.

Knowing how phentermine acts in your system and for how long can help you avoid drug interactions and side effects. It's also important to note that phentermine has the potential to become addicting.

Adipex can be detected in the blood, urine, and hair. The SAMHSA amphetamine test on the 5-panel urine test is the most common test administered. Hair follicle tests and blood tests are less common but may also be used.

Adipex detection times

 Verywell / Gary Ferster 

How Long Does It Take to Feel Effects?

Phentermine acts similarly to amphetamines by suppressing the appetite, stimulating the central nervous system, and elevating blood pressure. Phentermine is absorbed through the gastrointestinal tract and reaches peak concentrations in the blood in three to 4.4 hours, by which time you should start feeling the effects, alerting you that the drug is working.

The short-term effects of Adipex may include:

  • Decreased appetite
  • Euphoria
  • Hyperactivity
  • Anxiety, irritability
  • Chest pain

It is taken either as an extended-release capsule, which is taken once per day in the morning, or tablets, which are taken a half hour before meals, three times per day.

Phentermine increases levels of norepinephrine, dopamine, and serotonin, producing an effect similar to amphetamine. For this reason, phentermine is a controlled substance and only available legally via a prescription.

How Long Does Adipex Last?

Adipex has a half-life of about 20 hours, which is the time it takes for half of the dosage to be metabolized by the liver or excreted unchanged in the urine. It takes five to six half-lives for Adipex to be completely eliminated from your system.

A typical dose of Adipex is one 37.5mg capsule in the morning before breakfast. Adipex capsules contain the active ingredient phentermine hydrochloride, of which 70% to 80% is excreted in the urine unchanged. The rest undergoes N-oxidation and N-hydroxylation to form corresponding metabolites.


Adipex is detected via blood test for up to 24 hours.


Adipex is detected for one to four days by the SAMSHA 5-panel's amphetamine test. Urine tests are the most common, so it's fairly safe to stop taking Adipex a week before the drug test and expect for it to not show up on the test.


The hair follicle drug test can detect Adipex for several months.

False Positive Testing

Since phentermine has a similar structure to amphetamines, it can turn up a false positive on urine drug screens for amphetamines or MDMA, as confirmed by a 2016 study. A positive result will signal the need for a confirmatory test, which will then show that it was phentermine and not amphetamine or methamphetamine. The original result will then be ruled a false positive.

If you have a prescription for phentermine, you should disclose that to the testing lab so your results can be appropriately interpreted.

Factors That Affect Detection Time

The amount of phentermine detected in tests and how long it stays in your system is dependent on a number of factors.


Metabolism of Adipex may take longer in an older person.


If you are taking more than the recommended dose and using it off-label, Adipex may stay in your system longer.


A person of average build and metabolism can expect phentermine to be present in urine for three to five days. It may be longer for an older person

How to Get Adipex Out of Your System

It is recommended to taper Adipex use rather than immediately stopping. Adipex will generally be out of your system about two days after the last time you take it.

Phentermine is stored in body fat and burning fat may support elimination. That said, there is no healthy way to speed up this process. If you are expecting to be tested for Adipex, disclose that you are taking it or stop using it prior to being tested. People prescribed Adipex for weight loss often expect to take it for several weeks. Plan ahead.

Keep in mind that if you are using the drug as prescribed by your doctor, it is not a problem for Adipex to show up on a drug test.

Symptoms of Overdose

Phentermine has stimulant properties and can impact neurotransmitters including dopamine, norepinephrine, and serotonin. This is very similar to amphetamines. For this reason, prescriptions are tightly regulated and often only allowed for short periods of time. It is not recommended for people with a history of stimulant use disorder.

Some of the serious side effects of Adipex include increased blood pressure, heart palpitations, restlessness, dizziness, tremor, insomnia, shortness of breath, chest pain, and swelling of the legs and ankles.

Adipex overdose symptoms, on the other hand, may include:

  • Confusion
  • Panic
  • Hallucinations
  • Extreme restlessness
  • Nausea
  • Vomiting
  • Diarrhea
  • Stomach cramps
  • Feeling tired or depressed
  • Irregular heartbeats
  • Weak pulse
  • Seizure
  • Slow breathing (breathing may stop)

If you think someone has overdosed on any phentermine medication, call the poison control center at 1-800-222-1222. If the victim has collapsed or is not breathing, call 911 immediately.

Drug Interactions

It is important that you discuss all of your medications and supplements with your doctor to avoid drug interactions. Drugs of special concern include those used to treat depression and similar conditions, including:

Your doctor should also be made aware of any previous heart conditions, including high blood pressure and arteriosclerosis, or if you have hyperthyroidism. It's also best to avoid using alcohol while taking Adipex as the combination can intensify the effects.

Getting Help

Withdrawal symptoms from Adipex use tend to peak within the first couple of days and diminish over the next week. Common withdrawal symptoms from Adipex include:

  • Headaches
  • Lack of energy
  • High blood pressure
  • Diarrhea
  • Stomach pain
  • Vomiting
  • Menstrual irregularities
  • Insomnia
  • Irritability
  • Memory loss
  • Heart palpitations

If you are taking Adipex regularly at a high dose and stop taking it too soon, you can be at risk for more serious problems, including:

  • Congestive heart failure
  • Cardiac arrest
  • Strokes
  • Tremors
  • Seizures
  • Nerve damage

Adipex and other phentermine-containing medications may be available by prescription and can help with your weight loss goals. However, it is important to realize that there are potential risks involved, including developing a dependence and even a stimulant use disorder.

It could become difficult for you to concentrate without taking Adipex. You could enter into unhealthy cycles of taking more and less of the drug that put dangerous strain on your cardiovascular system.

Cravings for Adipex that end up interfering with your everyday life are cause for speaking with a mental health professional. Counseling or group therapy can help you understand the reasons for your Adipex use and develop alternatives and strategies to slowly stop using. Harm reduction psychotherapy and abstinence-based programs are both available depending on what you feel is best for you.

If you or a loved one is experiencing symptoms of Adipex addiction, help is available. Treatment providers can be found using the Substance Abuse and Mental Health Services Administration's Behavioral Health treatment services locator.

Was this page helpful?
2 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Marin SJ, Doyle K, Chang A, Concheiro-Guisan M, Huestis MA, Johnson-Davis KL. One hundred false-positive amphetamine specimens characterized by liquid chromatography time-of-flight mass spectrometry. J Anal Toxicol. 2016;40(1):37-42. doi:10.1093/jat/bkv101

  2. U.S. National Library of Medicine. Phentermine Hydrochloride. Updated October 19, 2018.

Additional Reading
  • American Association for Clinical Chemistry. Drug abuse testing. Updated November 20, 2019.

  • U.S. National Library of Medicine. Phentermine. Updated May 15, 2017.