How Long Adipex (Phentermine) Stays Your System

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

Phentermine is also the active ingredient in Fastin, Ionamin, and Zantryl and is one of the two ingredients in Qsymia. These medications are the most prescribed diet pills on the market. Also, because they 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.

How Phentermine Acts

Phentermine acts similarly to amphetamines in suppressing the appetite, stimulating the central nervous system, and elevating blood pressure. It is taken as a tablet or an extended-release capsule, with dosage and timing depending on the form. The extended-release capsule is usually taken once per day in the morning, while the tablets are taken a half hour before meals, three times per day.

After a dosage, phentermine is absorbed through the gastrointestinal tract and reaches peak concentrations in the blood in three to 4.4 hours. It has a half-life of about 25 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 drugs to be almost completely eliminated from your system.


Phentermine has stimulant properties and can increase hormone levels, including dopamine, norepinephrine, and serotonin. This is very similar to amphetamines, which is why there is a risk for addiction. For these reasons, prescriptions are tightly regulated and often only allowed for short periods of time. Additionally, it is not recommended for people with a history of drug abuse.

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. This includes monoamine oxidase inhibitors (MAOIs), such as Marplan and Nardil, and selective serotonin reuptake inhibitors (SSRIs), such as Prozac, Luvox, Paxil, and Zoloft.

Guanethidine and insulin medications for weight loss and depression are also of concern. If you require insulin for diabetes, you may need to make adjustments to it while taking phentermine.

Some people are allergic to phentermine or the tablet's ingredients. Your pharmacist can provide a list of the ingredients, which would be wise to review if you have allergic reactions to certain things.

Adipex isn't recommended for people over the age of 65. Your doctor should also be made aware of any previous heart conditions, including high blood pressure and arteriosclerosis, or if you have hyperthyroidism.

Phentermine is not recommended for women who are pregnant or plan to become pregnant. If you are breastfeeding, it can pass into your milk and might affect your baby.

It's best to avoid using alcohol while taking Adipex as it can make the side effects worse.

Side Effects

As with many medications, phentermine can cause side effects. You may experience dry mouth or an unpleasant taste. It's also possible to experience diarrhea, constipation, and vomiting.

It is important that you do not crush or cut the extended-release capsule as this will release more medication at once. Some of the serious side effects of Adipex include:

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.

Phentermine on Drug Tests

Since phentermine has a similar structure to amphetamines, a urine drug screen can test positive for amphetamines. This 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. Phentermine is likely to be detected in the urine for one to four days after a dose, up to 24 hours in the blood, and up to one month in hair tests.

The amount of phentermine detected in tests and how long it stays in your system is dependent on a number of factors. These include your age, the dose and how long you've been taking it, and your metabolism. For instance, a person of average build and metabolism can expect phentermine to be present in urine for three to five days. It may be faster for someone of a small build or longer for an older person.

A Word From Verywell

Apidex and other phentermine medications may be available by prescription and help with your weight loss goals. However, it is important to realize that there are potential risks involved with taking it. Be sure to have a good discussion with your doctor to see if it's right for you and avoid buying it on the street, the potential harm is too great.

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Article Sources

  1. National Institute of Health. Phentermine Hydrochloride. U.S. National Library of Medicine: DailyMed. 2017.

  2. Lonneman DJ, Rey JA, Mckee BD. Phentermine/Topiramate extended-release capsules (qsymia) for weight loss. P T. 2013;38(8):446-52.

  3. University of Rochester Medical Center. Amphetamine Screen (Urine). Health Encyclopedia.

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