You aren't really going to find a definitive answer, because the length of time it can still show up on a test will vary according to what you took, how much of it, and how long you were taking it. Also if it was an extended release for of a prescription drug, regular release, or instant release. Even illegal drugs will vary according to how strong they were, how much you used, and how often you used them in the period of time before your test.
As to the rest, unless someone is spending a fortune for very intense tests, the normal ones usually just show the usage class. So any narcotic would register, they wouldn't know if it was
Morphine or
Fentanyl. However, if you were majorly abusing, by taking way more than what you were supposed to, that will show on the level of the drug in your system.
On average, if you haven't been going crazy with them, you usually have the detectable amounts out of your system in approximately 3 days.
Wouldn't it just be smarted not to play around with other stuff when you have to have these tests? Then you wouldn't have to worry about this at all.
However, if you have a prescription for the Fentanyl pops, and you report it, that will usually negate any readings they get on the narcotics part of the test, since they just assume they will get a positive. Most testing places will just skip that part of the test, as long as you prove, or have your doctor verify that you have the prescription.
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My information is not guaranteed correct. I do not get them right all the time, but I do enjoy the hunt~