Question regarding drug testing and this pill I took?

The other thing to consider is that drug testing is looking for "abuse" levels of drugs in your system. If it is simply a qualitative positive vs negative result, then you must have above a certain testing threshold level that is considered abuse level by the performing lab. If it is above this level, then the result is positive. If it is a quantitative result (like a specified concentration such as X mcg/dL), then the lab with have a reference range for what is a normal therapeutic level and anything above this is considered positive and abuse of the drug. In general, illegal drugs get qualitative results and legal prescription drugs get quantitative results.

So in all likelihood, taking a single percocet will not get you a positive result on a drug test perform 3 days later. The half-life of the opioid ingredient in percocet is less than 5 hours in normal healthy people. That means your body clears half of the remaining drug out of your body every 5 hours. After 3 days, the amount left in your body is not likely to break the testing thresholds considered to be abusive of the medication.

Generally, depending on your residual, usually 3 to 5 days and you're okay. Prescriptions are usually good for a year. Check the date it was filled if its within a year your prescription is still good and there should be no worries. I wouldn't mention it even though you may be in the right but on the other hand could raise suspicions. If your prescription is out of date that could cause some problems just because you have a headache doesn't give you the right to take illegal drugs it still against the law.