The President is the highest elected position in the U.S. and therefore should be subject to the same employment requirements as all federal employees.
When Ronald Reagan signed Executive Executive Order 12564 in 1986 -- for a Drug-Free Workplace -- he wrote that "federal employees who use illegal drugs, on or off duty, tend to be less productive, less reliable, and prone to greater absenteeism than their fellow employees who do not use illegal drugs . . . The use of illegal drugs, on or off duty, by Federal employees impairs the efficiency of Federal departments and agencies (and) undermines public confidence in them."
In no other federal position could the issue of drug use be more consequential and we should therefore test for it.