How Long Adipex (Phentermine) Stays Your System

Adipex May Give a False Positive Urine Drug Test

Adipex 37.5 mg
Adipex (phentermine) is a diet pill. U.S. Drug Enforcement Administration

Adipex (phentermine) is a medication given to assist people with weight loss by decreasing appetite. It is usually prescribed for 3 to 6 weeks for 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. Knowing how phentermine acts in your system and for how long can help you avoid drug interactions and side effects.

How Phentermine Acts in Your System

Phentermine acts similarly to amphetamines in suppressing the appetite, stimulating the central nervous system, and elevating blood pressure. It is taken in 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, which the tablets are taken a half hour before meals, three times per day.

After a dosage, it is absorbed through the gastrointestinal tract and reaches peak concentrations in the blood in 3 to 4.4 hours. It has a half-life of about 25 hours, which is the time in which half of the dosage has been 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.

Discuss all of your medications and supplements with your doctor to avoid drug interactions. Drugs of special concern for interactions include monoamine oxidase inhibitors, Prozac, Luvox, guanethidine, insulin medications for weight loss and depression, Paxil, and Zoloft.

Adipex isn't recommended for people over the age of 65. It also isn't 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 alcohol can make the side effects worse.

Will Phentermine Show as Positive on a Urine Drug Test?

Phentermine has a similar structure to amphetamines, which share its ability to suppress appetite. On a urine drug screen, phentermine can turn the screen 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 is then ruled to be 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. Adipex is likely to be detected in the urine for 1 to 4 days after a dose, but this will vary from person to person depending on many factors.

It Is Important to Avoid Adipex's Severe Side-Effects

Don't crush or cut the extended-release capsule as that will release more medication at once. Some of the serious side-effects of Adipex include:

If you think someone has overdosed on Adipex, call the poison control center at 1-800-222-1222. If the victim has collapsed or is not breathing, call 9-1-1.


American Association for Clinical Chemistry "Drugs of Abuse Testing." Lab Tests Online. Revised 2 January 2013.

Phentermine. MedlinePlus NIH.

Phentermine Hydrochloride. DailMed. NIH.

What is Pentermine, and will it cause a positive result on a urine drug test? Human Performance Resource Center.