Every specimen type has a characteristic detection window. Oral fluid detects parent drugs within minutes of use and for roughly 24–48 hours — which is why it excels at post-accident and reasonable-suspicion testing. Urine detects drug metabolites for about 1–3 days for most substances (cocaine, opioids, amphetamines), though marijuana metabolites can persist for weeks in frequent users. Hair testing captures roughly 90 days of history but misses use within the last week or so, since drug-containing hair has not yet grown above the scalp. No method measures current impairment — a positive result shows exposure within the window, not intoxication at the time of the test. Choosing the right window for the testing reason (pre-employment, random, post-accident) is a core program design decision.