A blood alcohol content (BAC) of 0.08% is the threshold for drunk driving. States have different rules for commercial drivers, who might be subject to a 0.04% maximum. You can also be charged with a DUI for other substances, such as cannabis or prescription medication. Though most states do not have measurements for other recreational drugs or medications, such as opiates, law enforcement officers can arrest someone unable to operate a vehicle with the normal amount of care.