How far back does a blood test go concerning drugs?
This will entirely depend on the half life of the substance consumed; some may be about 2 days while several may be much longer. For instance, if someone is/was a frequent user, it can show up for several months.
It depends on what drug it is, some stay in your system longer than others.Valium and its metabolites will take about 14 days to leave your system. THC and its metabolites will take about 28 days.