Driving under the influence (DUI) generally refers to a person who has physical control of a vehicle while that person's judgment is impaired by any type of drug (legal or illegal), including alcohol and prescription medication. A vehicle can be defined as a car, a motorcycle, a boat, a bus, a truck, a snowmobile, or an ATV depending on the state law for the state in which the arrest is made. Most states have laws which specifically address drivers of buses or commercial vehicles in addition to laws addressing drivers of passenger vehicles.
Driving under the influence is a criminal charge and the laws regarding driving under the influence vary from state to state. The laws vary with regard to the amount of alcohol or drug in a person's system which defines influence, or intoxication as well as the arrest procedures and possible penalties. All states have some type of statute dealing with this criminal offense. In addition to DUI, there is DWI which is typically referred to as "Driving While Impaired." The difference in the two offenses is usually the amount of alcohol or drug present in a person's system at the time of arrest. Some states also have specific laws to deal with drivers who have a combination of drugs and alcohol in their systems at the time of arrest.
You can find all DUI state laws at each state's website for statutes. Cornell University Law School hosts a free website with links to each state's statutes by topic. If you would like to read the actual law and a list of criminal codes by state, go to http://topics.law.cornell.edu/wex/table_criminal_code .
Most states define "driving" as operating or having actual physical control of a vehicle. A passenger who is intoxicated could not be charged with DUI. Typically the person charged is the driver of a vehicle. If a driver were somehow incapable of driving and the passenger took control of the vehicle while under the influence of drugs or alcohol, then the passenger would have actual physical control of the vehicle and could be charged with DUI. If a police officer cannot prove who had actual physical control of a vehicle, a charge cannot be made.
In order to have sufficient proof of "driving" a person must be in a vehicle which is being "operated." If a person happened to be sitting in the driver's seat of a parked car and was intoxicated, that person could not be charged with a DUI. A running engine would not be enough to prove someone was "driving under the influence." There is no criminal offense of "intent to drive while impaired", so for a person to be charged, he must actually be operating a vehicle while impaired by drugs or alcohol.
The legal definition of drunk varies from state to state but all states measure "drunk" by testing the level of alcohol, or other drug, in a person's blood, urine or breath. Allowable methods of testing vary from state to state. Most state laws establish that a level of .08 percent blood alcohol concentration is enough to be charged with Driving Under the Influence. Charges and penalties increase in severity as the level of blood alcohol concentration increases. Again, states vary in the level increases which affect changes in penalty or charge. In Arizona, anyone with a blood alcohol concentration of .20 or more can be charged with a felony rather than a misdemeanor and will be subject to automatic increases in jail time and fines.
"Under the Influence" does not mean that a person is drunk. It means that a driver's ability to drive is impaired by any amount of drug or alcohol. Even some prescription medication can make a person so drowsy that it impairs his ability to safely operate a vehicle. A combination of medication and alcohol is often a cause of impairment for drivers. In most states, a driver under the legal drinking age can be charged with a DUI if he has any amount of alcohol in his system, even just a trace amount.
While all state laws vary in language, most include language which defines illegal drug use as that which a driver has taken with the purpose of causing intoxication, or which distorts the driver's ability to see, hear or think correctly. As an example, according to the Florida law "It is unlawful for any person to inhale or ingest, or to possess with intent to breathe, inhale, or drink, any compound, liquid, or chemical containing toluol, hexane, trichloroethylene, acetone, toluene, ethyl acetate, methyl ethyl ketone, trichloroethane, isopropanol, methyl isobutyl ketone, ethylene glycol monomethyl ether acetate, cyclohexanone, nitrous oxide, diethyl ether, alkyl nitrites (butyl nitrite), or any similar substance for the purpose of inducing a condition of intoxication or which distorts or disturbs the auditory, visual, or mental processes. This section does not apply to the possession and use of these substances as part of the care or treatment of a disease or injury by a [licensed practitioner]." Essentially, any drug smoked or ingested which impairs a driver's ability to safely operate a vehicle can be cause for a charge of "driving under the influence" or "driving while impaired."
While over-the-counter or legal prescription medication is legal to take, it is not legal to drive if your senses are impaired for any reason. Legal drugs can impair a person's ability to see, hear or think clearly and a person may be charged with driving under the influence if he is driving erratically or if he does not pass the breath, blood or urine tests. A person taking legal medication can argue that fact as a defense in court. A prescription, a letter from a doctor, or other evidence that the medication was necessary and perhaps that the driving was also necessary will assist in convincing the court to drop the charges against you.