Phentermine can be detected on urine drug tests if it is being tested for. In addition, phentermine, which is closely related to amphetamine, has been reported to cause false-positive tests for methamphetamine.
Based on the pharmacokinetic profile of phentermine, it can last in your body for nearly 5 days as the half-life of the drug is relatively long, around 19-24 hours.
In terms of drug testing specifically, most institutes allow you to give them a list of prescription medications when a drug screen comes back positive, so when you are able to provide a prescription for phentermine they likely will report back to whoever ordered the drug screen that you are clean.
Phentermine is a stimulant that suppresses appetite and is used to treat obesity. This is a controlled substance, so it can only be obtained by a prescription in the United States. This medication is typically dosed 1-2 times a day and this is due to the fact that this medication has a long duration of action in the body (around 8-14 hours) and a single dose can stay in the body for upwards of 100 hours.
Phentermine Drug Testing
On most five-panel drug screening tests, they will test for amphetamines. When a patient does test positive for a substance, most drug screening facilities will allow the patient to provide prescription medication lists as there are a number of prescription medications (phentermine included) that will cause a false positive.
When a prescription for a medication is provided they typically amend the drug test to show that there is, in fact, no positive result when reporting to the party who ordered the test.
Based on the time frame you provided (2 days), it will most likely take longer than that for phentermine to leave your system and you likely will have a positive drug screen for some form of amphetamine (e.g. methamphetamine). It may be a good idea to determine up front how to provide the drug screening institution with a list of your prescription medications so that they are better able to evaluate your drug test.