A pilot should not fly if they have had any alcohol within eight hours of the flight, are under the influence of alcohol or have a current blood alcohol level of .04.
In addition to alcohol, a pilot should not be under the influence of any drug that affects the his or her faculties in any way contrary to safety. Except in an emergency, no pilot of a civil aircraft may allow a person who appears to be intoxicated or who demonstrates by manner or physical indications that the individual is under the influence of drugs (except a medical patient under proper care) to be carried in that aircraft.
A pilot certificate or rating can also be denied and/or revoked if a pilot is convicted of operating a motor vehicle under the influence of alcohol or drugs. In addition, pilot certificate or rating can also be denied and/or revoked if he or she is convicted of any Federal or State statute relating to the growing, processing, manufacture, sale, disposition, possession, transportation, or importation of narcotic drugs, marijuana or depressant or stimulant drugs or substances.
14 CFR 61.15 Offenses involving alcohol or drugs
14 CFR 91.17 Alcohol or drugs