> Cannibis can be in your system for weeks or months

and the levels of active compounds[1] in somebody recently using, vs someone months away from use look very different, and their nature will generally work to the benefit of the defendant when reviewed by a competent forensic toxicologist.

[1] - not a yes/no from shitty screening methods; that kind of information comes off of quarter-million dollar mass spectrometers. they should be able to separate all of the inactive and active compounds from each other, enumerate them and list their exact concentrations in the blood.