The contribution of undertested drugs in DUID


Impaired driving is often accompanied by alcohol consumption and frequently results in crashes, injuries and deaths. According to the National Highway Traffic Safety Administration, one person was killed every 39 minutes in an alcohol-related crash in 2021.1 But alcohol isn’t the only concern; The use of illicit drugs, legalized drugs like cannabis, and abuse of prescription medications can also impair a driver’s abilities. In 2022, an estimated 13.6 million people have driven under the influence of illicit drugs in the previous year.2

In 2007, the National Safety Council (NSC) introduced standardization of the scope and limits of testing for drunk driving cases and traffic fatalities to improve testing consistency. Since 2013, it has recommended that forensic toxicology laboratories regularly analyze blood for 35 of the most commonly encountered drugs and metabolites. Called Level I drugs, they are now part of the testing standards in many forensic toxicology laboratories.3 Additionally, these compounds can be detected and confirmed with commonly used analytical instruments.

NSC also created a second category of drugs with significant impairment potential, called Tier II drugs. These drugs include new and emerging psychoactive substances, prescription drugs, and traditional drugs with limited or regional prevalence, many of which require advanced instrumentation for detection. Most laboratories test Tier I drugs, but only test certain Tier II drugs when regionally relevant. Therefore, the frequency and types of Tier II substances that contribute to drug-impaired driving incidents and fatal crashes are not well understood.

NIJ-funded researchers at the Forensic Science Research and Education Center examined blood samples from more than 2,500 cases of driving under the influence of drugs (DUID). The goal was to provide a detailed picture of Level I and Level II medications that contribute to impaired driving cases and compare the results to the NSC’s recommended testing ranges. Researchers also analyzed the presence of drugs at various blood alcohol concentrations to assess the operational impact of different testing thresholds and stop limit testing.

What is Stop Limit Test?

If a sample meets or exceeds a predetermined blood alcohol concentration threshold, some laboratories will not perform any additional drug testing. This threshold is most often 0.08% or 0.10%.4 In the United States, in all states, the legal blood alcohol limit is 0.08%. Laboratories that adhere to this practice will not detect other drugs that may cause or contribute to driving impairment.

These stop limit tests may undermine an overall understanding of the involvement of drugs in impaired driving. Why do so many laboratories use it?

  • Toxicology laboratories have limited budgets and resources.
  • Impaired driving can be explained solely by blood alcohol level.
  • The lack of enhanced penalties for drug use means there is no need to measure beyond blood alcohol content.
  • Agencies that use laboratory services have requested this limit.

National Security Council recommendations are supported

Researchers estimated how often drugs contribute to the national DUID problem by testing 2,514 cases using a spectrum of 850 therapeutic, abused and emerging drugs. They examined anonymized blood samples randomly selected from a group of suspected drunk driving cases. Samples were collected from NMS Labs in Horsham, Pennsylvania between 2017 and 2020.

Of the 2,514 suspected DUID cases examined:

  • The overall positivity rate for drugs (Tier I or Tier II drugs) was 79%, almost double the 40% positive for alcohol.
  • A smaller proportion of cases (23%) tested positive for both drugs and alcohol.
  • Only 17% of cases tested positive for alcohol alone.
  • Natural cannabinoids experienced a statistically significant increase in positivity over the four years.

Consumption of alcohol in combination with drugs from multiple categories was common, as was combined use of multiple drugs. THC (the main psychoactive component of marijuana) was most commonly found with ethanol (n = 359), and it was frequently found with amphetamine/methamphetamine (n = 146).

Samples with a blood alcohol level of 0.08% or more and also positive for Level I or Level II drugs were collected in 19% of cases (n = 478). Cases with a blood alcohol level of 0.10% (the threshold most frequently used by toxicology laboratories) also tested positive for Level I or II drugs 17.3% of the time (n = 434). This suggests that laboratories employing stop-limit tests may be missing many drug-positive cases.

“Limiting testing based on blood alcohol results excludes information about drug involvement in many cases and leads to underreporting of drug contributions to impaired driving,” said Mandi Moore , one of the researchers involved in the study.

Research supported the NSC’s recommendations for Level I and II testing. Level I drugs were found in 73% of suspected impaired driving cases, while only 3% contained only Level II drugs. This suggests that Level I testing captures the vast majority of drug-related DUID cases. However, some level II drugs (diphenhydramine, gabapentin, hydroxyzine and two new psychoactive substances) were detected as often or more often than some level I drugs, which could indicate their increased prevalence and the need to re-examine the lines guidelines.

Limitations of the study

The cases used in this analysis were exclusively from Pennsylvania. Therefore, they provide a geographically limited overview rather than a comprehensive characterization of the entire U.S. population. However, the sample size of more than 2,500 cases was “suitable to achieve the research objectives set out” by the researchers.

Because Level II psychoactive substances and new psychoactive substances were found at relatively low frequencies, researchers did not develop and validate additional confirmatory methods as they had previously planned.

Fill in the details of the general situation

This work raises awareness about drugs that laboratories are less likely to test and the role of laboratories in solving the DUID problem. This also shows how often DUID cases involve drugs other than alcohol. Although stopping limit testing may be warranted, data on alcohol and drug use provide the clearest picture of the contributing factors of DUID. Current estimates of drug frequency in DUID are likely inaccurate, and actual use is likely to be higher than previously thought due to testing discontinuation limitations. Equipping laboratories with sufficient resources could encourage them to eliminate stopping limit testing.

About this article

The work described in this article was supported by NIJ award number 2020-DQ-BX-0009, awarded to the Frederic Rieders Family Renaissance Foundation.

This article is based on the beneficiary’s report Assessing the contribution of emerging and undertested drugs to drug-impaired driving (pdf, 26 pages), by Amanda LA Mohr and Barry Logan, Center for Forensic Sciences Research and Education (CFSRE) of the Frederic Rieders Family Renaissance Foundation.

Republished courtesy of the National Institute of Justice.


(1), accessed January 29, 2024,

(2) Some illicit drugs include the use of marijuana, cocaine (including crack), heroin, hallucinogens, inhalants, or methamphetamine. For more information, see “Table 8.35A” in the 2022 NSDUH Detailed Tables, Substance Abuse and Mental Health Services Administration, /NSDUHDetailedTabs2022/NSDUHDetailedTabs2022/ NSDUHDetTabsSect8pe2022.htm#tab8.35a.

(3) ANSO/ASB 120 standard.

(4) Amanda D’Orazio, Amada Mohr, and Barry Logan, “Updated Recommendations for Drug Testing in DUID Investigations and Traffic Fatalities, Toxicology Laboratory Investigation,” Willow Grove, PA: Forensic Science Research and Education Center of the Frederic Rieders Family Foundation, June 28, 2020,