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

Adipex in Your Blood, Urine, & Hair

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What is Phentermine?

Adipex (phentermine) is a prescription medication given to assist people with weight loss, along with exercise and a low-calorie diet. It works by decreasing appetite and, while only approved for short-term use of a few weeks, more than half of patients stay on phentermine for four months or more.

Phentermine is also the active ingredient in other prescription drugs, including Fastin, Ionamin, and Zantryl, and one of two ingredients in Qsymia. Medications containing phentermine are the most prescribed diet pills on the market, accounting for 74% of the market share.

These drugs are similar to amphetamines and act as a stimulant, increasing norepinephrine, dopamine, and serotonin levels. For this reason, phentermine is a controlled substance and has the potential for abuse and dependence. It is only available legally via a prescription.

Knowing how phentermine acts in your system, and for how long, can help you avoid drug interactions and side effects. It can also help you understand whether it will appear on a drug test as Adipex can be detected in the blood, urine, and hair.

Adipex detection times

 Verywell / Gary Ferster 

How Long Does Phentermine Take to Feel Effects?

Phentermine acts similarly to amphetamines in the body by suppressing appetite, stimulating the central nervous system, and elevating blood pressure. It reaches peak concentrations in the blood in three to 4.4 hours. This means that the effects of phentermine should kick in by this time, alerting you that the drug is working.

Short-term effects of Adipex can include:

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

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

Phentermine pills can stay in your system for quite some time, so they can show up on a drug test. Urine tests are the most common drug tests administered but hair follicle tests and blood tests may also be used.

In Blood

Adipex can be detected via a blood test for up to 24 hours.

In Urine

Adipex can be detected for one to four days in the urine. Since urine tests are the most common, stopping Adipex a week before the drug test can help it to not show up on the test.

In Hair

A 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, as confirmed in a 2016 study. A positive result signals the need for a confirmatory test, which then shows that phentermine was detected and not amphetamine or methamphetamine. The original result can then be ruled a false positive.

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

Factors That Affect Phentermine Detection Time

The amount of phentermine detected in a drug test and the length of time it stays in your system are dependent on a number of factors.


Adipex elimination may take longer in an older person due to decreased kidney function with age.


If you are taking more than the recommended dose of phentermine, this drug may stay in your system longer.


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

How to Get Adipex Out of Your System

Phentermine will generally be out of your system within a few days after the last time you take it. No strategies have been found to increase its elimination any faster.

If you are expecting an upcoming drug test, either disclose that you are taking phentermine or stop using your Adipex prior to being tested. Since people prescribed phentermine for weight loss often take it for several weeks, if not months, planning ahead is important.

If you are using phentermine as prescribed by your doctor, it should not a problem for Adipex to show up on a drug test.

Side Effects of Phentermine

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.

People taking phentermine may experience side effects such as:

  • Constipation
  • Diarrhea
  • Dry mouth
  • Unpleasant taste in the mouth
  • Vomiting

Some of the potentially serious side effects of Adipex include:

  • Chest pain
  • Dizziness
  • Heart palpitations
  • Increased blood pressure
  • Insomnia
  • Restlessness
  • Shortness of breath
  • Tremors
  • Swelling of the legs and ankles

If you experience any of these serious phentermine side effects, reach out to your healthcare provider or seek medical attention immediately.

Symptoms of Overdose

If too much or too high a dose of Adipex is taken, it can lead to an overdose. Phentermine overdose symptoms may include:

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

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 healthcare provider to avoid drug interactions. Drugs of special concern when taking phentermine include those used to treat depression and similar conditions, including:

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

If you've had a history of drug abuse or misuse, it's important that your healthcare provider know this as well since phentermine use may be harmful in this situation.

Withdrawal Symptoms

When you stop taking phentermine, you may experience withdrawal. Adipex withdrawal symptoms tend to peak within the first couple of days and diminish over the next week. Adipex withdrawal symptoms can include:

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

If you are taking high doses of Adipex regularly and stop taking it too soon, you may be at risk for more serious problems, including:

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

Some research suggests that symptoms experienced after stopping phentermine abruptly aren't due to actual withdrawal from the drug, so this issue is somewhat unclear.

A Word From Verywell

Adipex and other phentermine-containing medications are available by prescription and, according to research involving 13,972 people, can contribute to greater weight loss. However, it is important to realize that there are potential risks involved with these types of drugs.

Your healthcare provider can help you determine if Adipex or other phentermine medications are right for you based on your conditions and health status.

10 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.
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  3. U.S. Food and Drug Administration. Adipex-P (phentermine hydrochloride USP).

  4. National Library of Medicine, National Center for Biotechnology Information. Phentermine.

  5. 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

  6. International Programme on Chemical Safety. Phentermine.

  7. U.S. National Library of Medicine. Phentermine.

  8. U.S. National Library of Medicine. Label: phentermine hydrochloride capsule.

  9. Hendricks EJ. Off-label drugs for weight management. Diabetes Metab Syndr Obes. 2017;10:223-234. doi:10.2147/DMSO.S95299

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Additional Reading
  • American Association for Clinical Chemistry. Drug testing.

By Buddy T
Buddy T is an anonymous writer and founding member of the Online Al-Anon Outreach Committee with decades of experience writing about alcoholism.