How a substance shows up in a drug
test or how long it can be detected is only a small part of the whole. Of greater concern is the plasma concentration of the drug, combined with the absorption and decay curves thereof, which is used to differentiate between legitimate use and illegitimate abuse of a drug.
To expand on that, it's not the presence of amphetamines in a drug test alone that indicates addiction
or abuse but rather the amounts of amphetamine
in various concentrations in your plasma (blood), how fast those concentrations were achieved (indicating ROA) as well as the frequency, estimated timespan and estimated last administration. All of these factors can be revealed by evaluating absorption and decay curves as measured by comparing various concentrations of a substance in your plasma and comparing it to the absorption rate, bioavailability and half-life
of the substance. Data that is already known for prescription drugs
and most popular illegal substances out there.
Or to state it as plain as possible, they can see from test results whether you swallowed, smoked, snorted or injected a drug, how much of the drug you have used thus, how often you used it and the last time you used it, more or less.
Which means if you have a prescription for Adderall
from your psychiatrist for ADHD or ADD, you would have nothing to worry about provided you were using your medication as prescribed. The test results will verify legitimate medical use. However, if you have a prescription for such medication but you're snorting
crushed pills, taking larger doses than prescribed or taking doses more frequently than prescribed, to name a few examples, then such abuse will be easily revealed in your test results. You would probably have a hard time explaining such behaviour.
Not having a legal prescription for Adderall or Ritalin would obviously raise alarm bells just by being present, regardless of quantity or any other considerations. The same would go for any illegal substances such as methamphetamine
because the use of such is prohibited, no matter how 'therapeutic'. The detection times of such substances varies depending on the person and the substance involved. For example methamphetamine can be detected as long as a week after the fact, in some frequent users who use higher than normal doses on average.
There is more detailed information about the most common substances regarding detection times and the subject of plasma concentration in this PDF document courtesy of labmed.yale.edu