A person is considered to be driving under the influence (DUI) when they operate a motor vehicle while under the influence of alcohol or any other controlled substance, such as marijuana and other illegal drugs, to the point that the person's normal faculties are impaired.
For alcohol, a person's normal faculties are considered impaired when their blood alcohol level (BAC) is 0.08 or higher. Some people may find themselves charged with a DUI when their BAC is below 0.08. This is because a impairment can affect a person at different consumption levels.
A person's impairment is not as easily determined when they have consumed a controlled substance, but if the substance is impairing the driver's faculties, they will be considered driving while under the influence.