Except in cases where the drug is being used as prescribed for a patient by a licensed physician, all persons entering in or on Federal property are prohibited from—

(a) Being under the influence, using or possessing any narcotic drugs, hallucinogens, marijuana, barbiturates, or amphetamines; or

(b) Operating a motor vehicle on the property while under the influence of alcoholic beverages, narcotic drugs, hallucinogens, marijuana, barbiturates, or amphetamines.