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Drug-Impaired Driving: The Contribution of Emerging and Undertested Drugs

NCJ Number
Date Published
May 2024

This article addresses the impacts of driving under the influence of illicit and prescription drugs (DUID) and laboratories' role in resolving the DUID problem; it provides a listing of Tier I and Tier II drugs and discusses what they are as well as the differences between the drugs in the two categories; it describes stop-limit testing and what it means for labs that practice it; and it discusses evidence in support of National Safety Council recommendations for Tier I and Tier II drug testing.


Impaired driving is often associated with alcohol use and frequently leads to accidents, injuries, and fatalities. But alcohol is not the only concern; the use of illicit drugs and abuse of prescription medications may also impair a driver’s abilities. In 2007, the National Safety Council (NSC) introduced testing scope and cutoff standardization for impaired driving cases and traffic fatalities to improve testing consistency. Since 2013, it has recommended that forensic toxicology labs regularly test blood for 35 of the most often encountered drugs and metabolites. Referred to as Tier I drugs, they are now included as a testing standard in many forensic toxicology labs. NSC also created a second drug category with significant impairment potential, termed Tier II drugs. These drugs include emerging novel psychoactive substances, prescription drugs, and traditional drugs of abuse with limited or regional prevalence, many of which require advanced instrumentation for detection. Most laboratories test for Tier I drugs, but only test for select Tier II drugs when they are regionally relevant. National Institute of Justice (NIJ)-funded researchers examined blood samples from over 2,500 driving under the influence of drugs (DUID) cases. The goal was to create a detailed picture of both Tier I and Tier II drugs that contribute to impaired driving cases and compare results to the NSC’s recommended testing scopes. Researchers also analyzed drug presence at various blood alcohol concentrations to assess the operational impact of different testing thresholds and stop limit testing. Stop limit testing is when some labs don’t perform any additional drug tests if a sample meets or exceeds a pre-determined blood alcohol concentration threshold.