Phentermine is a stimulant used for weight loss that is chemically and pharmacologically related to amphetamine. Phentermine will show up as amphetamine in a urine test for amphetamines.
Urine tests are the easiest and cheapest way for authorities to detect amphetamines and phentermine will be detectable within your urine within two to five hours of the first dose, much earlier if you take it regularly. The amount of time it remains detectable for depends on how much you have taken and how regularly you take it (the higher the dose or the more regularly you take it the longer it stays in your system), how hydrated you are (if you are dehydrated drugs tend to stay in your system longer), and your metabolism. In addition, your body size, your percentage body fat, your age, and the dosage form (for example extended release) also influence how long it is detectable in your body for.
Phentermine has a half-life of around 19 to 25 hours. The half-life is an indication of how long it takes the body to metabolize 50% of a drug. As a rule, a drug is considered “gone” or no longer present in the body after 5 to 6 half-lives. This means it will take at least five to seven days for ONE dose of phentermine to completely leave the body. Of course, this half-life is influenced by all the other factors listed above to an unknown extent.
For a drug test to record a negative result, the level of phentermine must be below detectable levels. These levels vary depending on the urine test used, but generally, the tests are not sensitive enough to detect extremely tiny amounts. One specialist laboratory test website considered the detection window for amphetamines was up to 3 days for occasional use and up to 10 days for regular, chronic use.
If your drug test records a positive result and it has been at least seven days since you have taken a dose of phentermine, ask for a retest. Be aware that decongestants such as ephedrine and some other medicines used to treat ADHD or narcolepsy will also test positive for amphetamines. If you have been prescribed phentermine legitimately, list it on your medication list and take the bottle with the label attached to the drug testing facility. If you have been taking phentermine or any other amphetamine recreationally, be aware that these medicines are incredibly addictive and can make driving, operating machinery, and other activities extremely difficult. You may wish to contact a drug treatment facility to help you get off them.