Drug-impaired driving, also called driving under the influence of drugs (DUID) or drug driving, is the operation of a motor vehicle while a psychoactive drug other than - or in addition to - alcohol reduces the driver's ability to drive safely. The conduct is a criminal or administrative offense in many countries.
Drugs implicated include illicit substances, prescribed and over-the-counter medicines, and combinations of those substances with alcohol. Laboratory and epidemiological studies have documented effects on attention, tracking, reaction time and risk-taking for several drug classes. The relationship between a measured concentration in blood or oral fluid and crash risk is much clearer for alcohol than for most other drugs.
The World Health Organization treats driving under the influence of alcohol or other psychoactive substances as a major crash risk factor. It notes that the increase in crash risk varies by substance; amphetamines, for example, have been associated with about a fivefold rise in fatal-crash risk relative to non-use. Roadside surveys measure how often drugs are present in drivers, not whether those drivers are impaired. In the 2013–2014 U.S. National Roadside Survey, about one in five sampled drivers tested positive for at least one drug; the European DRUID project found lower average rates in general traffic, with large regional differences and with alcohol still the substance most often found in seriously injured and killed drivers.
Legal systems use more than one method of proof. An impairment statute requires evidence that the drug affected driving. Zero tolerance and numeric statutes treat a laboratory result as the offense. Some jurisdictions allow a laboratory result to support an inference of impairment without making the number conclusive. Testing and reporting practices vary widely, which limits comparison of arrest and fatality statistics across places and years.
History Impaired-driving statutes began as alcohol offenses. Early 20th-century codes typically required proof that the driver was “intoxicated” or “under the influence,” judged from observed behavior. Chemical testing shifted that focus. Indiana and Maine adopted presumptive blood alcohol content levels in 1939; the Uniform Vehicle Code later incorporated similar chemical-test provisions. New York enacted the first U.S. implied consent law in 1952. Robert Borkenstein’s Breathalyzer (1954) made roadside alcohol measurement practical, and by 1973 every U.S. state had an implied-consent statute.
Drug-impaired driving was slower to acquire its own tools. Officers routinely encountered drivers who appeared impaired but registered little or no alcohol. In the early 1970s two Los Angeles Police Department sergeants, working with physicians and psychologists, developed a standardized evaluation that became the Drug Evaluation and Classification (DEC) protocol. The department formally recognized a Drug Recognition Expert (DRE) program in 1979. The National Highway Traffic Safety Administration funded multi-state pilots in 1987–1988; the International Association of Chiefs of Police later coordinated national and international expansion.
From the 1990s onward, jurisdictions added zero-tolerance or numeric drug rules on top of impairment offenses, first in parts of Europe and then more widely after oral-fluid screening became commercially available. Victoria, Australia, introduced random roadside oral-fluid testing in December 2004, modeled on random breath testing and initially targeting methamphetamine, MDMA and THC. Sweden adopted a scheduled-drug zero-tolerance rule in 1999; Norway set graded blood limits for twenty drugs in 2012, calibrated to degrees of alcohol impairment. Canada added regulated blood-drug concentrations in 2018. England and Wales added specified-drug concentration offenses in 2015. Germany replaced a near-zero judicial THC practice with a 3.5 ng/mL serum limit in 2024 after recreational cannabis reform.
Effects by drug class NHTSA's DEC program groups impairing drugs into seven categories used by Drug Recognition Experts: central nervous system (CNS) depressants, CNS stimulants, hallucinogens, dissociative anesthetics, narcotic analgesics, inhalants, and cannabis. Experimental and epidemiological work does not treat those categories as equal in crash risk.
Alcohol remains the benchmark. Crash risk rises in a reproducible way as blood alcohol content rises. In NHTSA's Virginia Beach case–control study, drivers at 0.08% BAC were about four times as likely to crash as sober drivers, and about twelve times as likely at 0.15%.
CNS depressants other than alcohol include benzodiazepines and Z-drugs such as zolpidem and zopiclone. They impair divided attention and increase lane-position variability in experimental studies. In DRUID roadside surveys they were among the medicines most often detected in northern Europe, typically in older daytime drivers. Benzodiazepines have also been among the most frequently detected medicines in some crash series when laboratories test for them.
Opioids, whether illicit or prescribed (the DEC "narcotic analgesic" category), can slow reaction time and produce drowsiness. Medicinal opioids appeared more often in northern European roadside samples than illicit opiates did. In the U.S. National Roadside Survey, prescription and over-the-counter medications that can affect driving were found in 7.3% of weekend nighttime drivers - a separate group from illegal-drug positives.
Stimulants such as cocaine and amphetamines can improve some laboratory measures of alertness at low doses and degrade judgment and impulse control; fatigue can follow the acute phase. DRUID associated amphetamines with elevated injury risk, especially in combination with other substances. Cocaine was the second most common illicit drug in several southern European roadside samples after cannabis.
Cannabis / delta-9-THC. A 2022 meta-analysis of 57 experimental driving studies found that cannabis impaired lateral control (lane weaving and excursions) and was associated with lower driving speed; combined cannabis and alcohol produced larger decrements than either substance alone, and cannabis effects were described as similar in magnitude to low blood-alcohol concentrations. A related meta-analytic review of acute Δ9-THC effects estimated that most driving-related cognitive skills recovered within about five hours after inhaling 20 mg of THC in occasional users, with oral doses taking longer; regular consumers showed smaller average deficits. Blood THC falls quickly after inhalation while effects may continue, and regular consumers can have residual blood THC after acute effects have ended. Systematic reviews have found no reliable linear relationship between a single blood-THC value and driving performance. Observational syntheses report low-certainty associations between cannabis use and crash fatality or injury; they do not resolve dose or timing.
Hallucinogens, dissociative anesthetics (phencyclidine, ketamine) and inhalants are less common in roadside samples but can produce marked perceptual and motor disturbance. Several countries set "any detectable level" limits for LSD, PCP, ketamine or heroin's 6-monoacetylmorphine rather than a numeric impairment threshold.
Combinations of alcohol with another drug, or of two drugs, produced some of the highest odds ratios for serious injury or death in DRUID's case–control analyses - higher than most single-drug findings except high BAC.
Epidemiology Presence is not impairment A positive toxicology test shows that an analyte was present above a laboratory cut-off in the specimen drawn. It does not by itself establish that the person was impaired at the time of driving, or that the drug caused a crash. NHTSA states that distinction in its roadside-survey and crash-risk reports.
Roadside surveys The 2013–2014 National Roadside Survey sampled volunteer drivers at 300 locations in the continental United States. Using oral fluid and blood, about 22% of both daytime and nighttime drivers tested positive for at least one of 98 drugs (illegal, prescription or over-the-counter). Delta-9-THC was the most frequent drug (8.7% of daytime and 12.7% of nighttime drivers). Among weekend nighttime drivers, 15.2% were positive for an illegal drug and 7.3% for a prescription or over-the-counter medication that can affect driving. On a panel comparable to 2007, nighttime drug prevalence rose from 16.3% to 20.1%. Over the same decades, weekend-night drivers at or above 0.08% BrAC fell from 7.5% in 1973 to 1.5% in 2013–2014.
The European DRUID project sampled about 50,000 drivers in 13 countries. Estimated prevalence in general traffic was 3.48% for alcohol (≥0.1 g/L), 1.90% for illicit drugs, 1.36% for selected medicines, 0.39% for drug–drug combinations and 0.37% for alcohol plus a drug or medicine. THC was the most common illicit drug in traffic overall. Benzodiazepines and medicinal opioids were more characteristic of northern Europe and of older female daytime drivers. Southern and western Europe had higher roadside rates of alcohol, cocaine and cannabis. Eastern European samples were lower on average.
Comparable random-sample figures are scarce for Latin America, Africa and much of Asia. Enforcement in those regions more often relies on observed impairment, crash investigation, or alcohol screening; oral-fluid programs are uncommon outside high-income jurisdictions.
Crash-involved drivers Among seriously injured or killed drivers in DRUID hospital and fatality studies, alcohol alone was the substance most often detected, followed by alcohol combined with another substance. Combined use was far more common in crash-involved drivers than in roadside traffic.
NHTSA's Virginia Beach case–control study compared more than 3,000 crash-involved drivers with 6,000 controls matched on location, time and direction of travel. Unadjusted odds of crash involvement were 1.21 for drivers positive for illegal drugs and 1.25 for THC. After adjustment for age, sex, ethnicity and alcohol, the drug associations were no longer statistically significant; alcohol's association remained large. The study was conducted in one metropolitan area, most crashes were non-fatal, and the drug measure was presence rather than dose or documented impairment.
Charged-driver and fatal-crash files Arrest files are not a sample of all drivers. Where states publish case-processing reports, the share of filings that include a drug test, and the share that are alcohol-only versus mixed, vary with laboratory capacity. Conviction patterns in a single state cannot be treated as national effects of a particular statutory wording.
The U.S. Fatality Analysis Reporting System (FARS) is a census of crashes with a death within 30 days. Its alcohol fields are widely used for national estimates. NHTSA has warned that FARS drug fields are not comparable: who is tested varies by state and by whether the driver died; laboratories use different matrices, panels and cut-offs; until a 2018 redesign the file stored at most three non-alcohol drugs under a hierarchy that under-counted combinations; missing drug results are not missing at random and are not imputed as missing BACs are; and a recorded positive test is presence, not impairment or causation. Year-to-year changes in "drug-positive fatal-crash drivers" therefore mix changes in use with changes in testing and reporting.
Detection and enforcement Roadside investigation