The effects of specific drugs on driving skills differ depending on how they act in the brain. For example, marijuana can slow reaction time, impair judgment of time and distance, and decrease coordination. Drivers who have used cocaine or methamphetamine can be aggressive and reckless when driving. Certain kinds of prescription medicines—including benzodiazepines and opioids—can cause drowsiness, dizziness, and impair cognitive functioning (thinking and judgment). All of these effects can increase the chances of vehicle accidents.

Research studies have shown negative effects of marijuana on drivers, including an increase in lane weaving, poor reaction time, and altered attention to the road. Use of alcohol with marijuana makes drivers more impaired, causing even more lane weaving. Some studies report that opioids can cause drowsiness and impair thinking and judgment. Other studies have found that being under the influence opioids while driving can double your risk of having a crash.

For these reasons, many states have laws making it a criminal offense to operate a vehicle under the influence of or while impaired or intoxicated by legal or illegal prescription or recreational drugs, for example—known as drugged driving or driving under the influence of drugs (DUID). And a driver’s use of a prescription medication is not a defense to operating a vehicle while impaired or intoxicated by the medication.

In many states the DUI/DWI statutes are broad enough to prosecute a driver who is under the influence of or impaired or intoxicated by prescription or recreational drugs—as well as alcohol or any other substance, including an inhalant with a vapor-releasing substance. And at least 15 states have per se (pronounced purr-say) drugged driving laws that make it a criminal offense to drive with any detectable amount of certain drugs in your system, or with a minimum prohibited amount of certain drugs, as provided in the state’s statutes. These states include Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Minnesota, Nevada, Ohio, Pennsylvania, Rhode Island, Utah, Virginia, and Wisconsin.

Drugged driving laws are generally located in a state’s statutes—often in the penal code or criminal code.

In Iowa, as in many other states, it is illegal to operate a vehicle while under the influence of drugs, whether they are legal or illegal, prescription or recreational. This includes substances that can impair driving abilities, such as marijuana, cocaine, methamphetamine, benzodiazepines, and opioids. The state's drugged driving laws are designed to prevent driving while a person's reaction time, judgment, coordination, or cognitive functioning is impaired due to drug use. Iowa is one of the states with 'per se' drugged driving laws, which make it a criminal offense to drive with any detectable amount of certain prohibited drugs in one's system. The use of prescription medication is not a valid defense against charges of operating a vehicle while impaired. These laws are typically found in the state's penal or criminal code, and they are enforced to reduce the risk of vehicle accidents caused by drug-impaired drivers.