The "legal limit" at which the law assumes a driver is impaired by alcohol is a blood or breath alcohol concentration of 0.08 percent. He may also be arrested and convicted of DUI, however, with a significantly lower BAC if the arresting officer believes she her evaluation of the motorist's condition shows that he is actually impaired. But what if the sheriff's deputy or police officer takes the alleged DUI offender back to the station for testing and finds that the driver consistently blows a very low number, like 0.01 or 0.02 percent, or even shows a BAC of 0.0 percent?
The average person might assume that he would be let go, on the basis that the arresting officer must have made an error. This is probably not as likely as the typical driver would like to believe. Instead, the motorist will be asked to submit to other testing by way of providing blood or even urine samples. This is in part because the police may be suspicious that the driver is impaired by a different substance, or another substance in combination with alcohol. Additionally, blood samples are generally perceived as showing the true amount of alcohol in a person's system more reliably than a breath test machine. Still, the results may show no alcohol.
A charge of DUI can be maintained if the motorist is believed to have been under the influence of drugs. Generally, this is taken to mean illegal drugs, but it can also mean prescription drugs, regardless of whether the motorist has a valid prescription, and even over-the-counter drugs. The over-the-counter drugs may especially be considered when found in tandem with another substance. For example, a moderate amount of alcohol combined with an allergy medication that causes drowsiness is likely to render a person impaired, even if there was no such intent. The blood and urine tests allow law enforcement agencies to find substances other than alcohol. No other drug, however, has a baseline level at which the law presumes intoxication. Prosecutors often operate on the assumption, then, that even small amounts of a drug such as cocaine could have caused a DUI if a police officer testifies that a motorist's driving appeared dangerous or abnormal.
