Law and imprisonment have no business whatsoever in this domain. Because drugs are bad. Because using them is illegal. Occasionally one encounters more sophisticated arguments. Drugs taken during pregnancy can damage the fetus.
Drug use imposes an unfair burden on government-subsidised health systems.
Establish a competency test for drivers that can be administered in the field. Forget blood alcohol levels, drug screens. Forget letting tired drivers off with a warning. If behavior directly endagers someone, make this behavior illegal. First, we need to decide whether we make this a crime.