In the US, it's generally only possession (not use) that's a crime. The main exception is while operating a vehicle, which is just about the only time the polic

In the US, it's generally only possession (not use) that's a crime. The main exception is while operating a vehicle, which is just about the only time the police will do blood testing. Hospitals will do a tox screen if they're taken in for some medical reason, but it becomes exactly that: a medical issue, not a legal one.