Your Experienced Legal Team | Se Habla Español 

DWI Defense: Understanding the Reliability Limitations of Standardized Field Sobriety Tests (SFSTs) in Marijuana Impairment Detection

As a DWI defense attorney, I often encounter clients who have been charged with driving while impaired by marijuana. While most people are familiar with the reliability of field sobriety tests (SFSTs) for detecting alcohol impairment, fewer understand how these tests fare when it comes to marijuana. This blog aims to shed light on the effectiveness and limitations of SFSTs in detecting marijuana impairment compared to alcohol.

What Are Standardized Field Sobriety Tests (SFSTs)?

Standardized Field Sobriety Tests (SFSTs) are a set of tests used by police officers to determine whether a driver is impaired. These tests include the Horizontal Gaze Nystagmus (HGN), Walk and Turn (WAT), and One Leg Stand (OLS). While these tests have been validated for alcohol impairment, their effectiveness in detecting marijuana impairment is a subject of ongoing research and debate.

Key Research Findings on Marijuana and SFSTs

To understand the reliability of SFSTs in detecting marijuana impairment, it's essential to look at the research. Several studies have examined this issue, each providing valuable insights into the limitations and potential inaccuracies of SFSTs when applied to marijuana impairment.

1. Papafotiou et al. (2005)

In a pivotal study published in Forensic Science International, Dr. Katherine Papafotiou and her team explored the effects of marijuana on driving performance and the accuracy of SFSTs in detecting impairment. The study involved subjects smoking marijuana cigarettes with varying THC concentrations (0%, 1.74%, and 2.93%).

  • Findings: The study found that driving performance was not significantly impaired 30 minutes after THC consumption but was impaired at 80 minutes. Interestingly, at 80 minutes post-consumption, 88.5% of those impaired in driving were correctly identified by SFSTs. However, a staggering 61.5% of non-impaired individuals were falsely classified as impaired by SFSTs.
  • Conclusion: This study suggests that SFSTs might be more sensitive to THC consumption than actual driving ability, as evidenced by the high false positive rate. The authors concluded that "the SFSTs provide a more sensitive measure of THC consumption than driving ability itself," highlighting a significant limitation in using SFSTs for marijuana impairment detection.

2. Downey et al. (2012)

Another important study examined the relative sensitivity of SFSTs in detecting cannabis use with and without alcohol. This research, published in Psychopharmacology, included cannabis doses of 1.8% and 3% THC and alcohol doses ranging from 0 to over 50 mg/100 mL.

  • Findings: The study concluded that the relative sensitivity of SFSTs in detecting drug usage was limited. Notably, there was no direct assessment of driving impairment included in this study.
  • Conclusion: The findings underscore the limitations of SFSTs in accurately detecting marijuana use, particularly when combined with alcohol.

3. Bosker et al. (2012)

A study by Bosker et al., published in Addiction, compared the effects of alcohol and marijuana on SFSTs in heavy cannabis users. This randomized, placebo-controlled study included three doses of alcohol to achieve BACs of 0, 0.50, and 0.70 g/dL, along with a standardized inhalation technique for administering a dose of 11% THC through marijuana cigarettes.

  • Findings: The study found that while the combination of THC and alcohol did relate to HGN, SFSTs were only mildly sensitive to impairment from cannabis in heavy users. The researchers noted that tolerance might have been an issue, as they used heavy cannabis users in their study.
  • Conclusion: This study suggests that tolerance among heavy users might influence the sensitivity of SFSTs, further complicating the detection of marijuana impairment.

4. Yoshizuka et al. (2014)

In a series of studies comparing SFSTs across various sedating and non-sedating drugs, Yoshizuka and colleagues demonstrated the limitations of SFSTs in detecting drug impairment. While these studies did not specifically involve marijuana or alcohol, they provided valuable insights into the reliability of SFSTs.

  • Findings: The studies demonstrated a 26% failure rate in SFSTs even without pharmacological perturbation, highlighting the potential for false positives.
  • Conclusion: The high false positive rate in these studies calls into question the reliability of SFSTs for detecting impairment, particularly for drugs other than alcohol.

5. Heishman et al. (1996)

Heishman and colleagues conducted a laboratory study evaluating the effects of ethanol, cocaine, and marijuana on SFSTs. The study involved 18 volunteers who were given varying doses of these substances.

  • Findings: The study found that police officers correctly identified the presence of drugs in only 51% of cases, a rate similar to chance.
  • Conclusion: Despite the presence of drug cues, the accuracy of impairment detection was poor, suggesting that SFSTs may not be reliable for detecting marijuana impairment.

6. Canadian Studies (Beirness et al. 2007, 2009)

In a comprehensive review of laboratory and field studies on Drug Recognition Expert (DRE) evaluations, Beirness and LeCavalier provided valuable insights into the accuracy of these tests.

  • Findings: Laboratory studies showed low accuracy in detecting drug classes and impairment, while field studies often relied on self-reported drug use and other cues.
  • Conclusion: The researchers concluded that DRE evaluations are not reliable for proving impairment but may detect drug use better than chance.

Implications for DWI Defense

The research findings discussed above have significant implications for defending clients against DWI charges involving marijuana. Here are some key points to consider:

  • High False Positive Rates: SFSTs may incorrectly identify non-impaired individuals as impaired, particularly when it comes to marijuana. This high rate of false positives can lead to wrongful arrests and convictions.
  • Sensitivity to THC: SFSTs might be more sensitive to the presence of THC rather than actual driving impairment. This means that a positive SFST result does not necessarily indicate that a person is unfit to drive.
  • Lack of Validation for Marijuana: Unlike alcohol, SFSTs lack robust validation for detecting marijuana impairment. This lack of validation raises questions about the reliability of these tests in marijuana-related DWI cases.


The limitations of SFSTs in accurately detecting marijuana impairment highlight the need for more reliable and validated testing methods. As a DWI defense attorney, it is crucial to be aware of these limitations and to use this knowledge to challenge the validity of SFST results in court.

If you or a loved one is facing DWI charges involving marijuana, it is essential to have a defense attorney who understands the nuances of SFSTs and the science behind impairment detection. At Zendeh Del & Associates, PLLC, we have the expertise and knowledge to provide a robust defense against DWI charges.

About Jonathan Zendeh Del

Jonathan Zendeh Del is a top-notch DWI defense attorney in Galveston, Texas. As the managing attorney at Zendeh Del & Associates, PLLC, the largest criminal defense law firm in Galveston County, Jonathan is recognized as a DWI Super Lawyer. He has received extensive specialized training in DWI defense, including studying the same materials the police are trained on. Jonathan's deep knowledge of field sobriety tests, blood science, and police procedures in DWI cases allows him to bring attention to alternative explanations and defense strategies in court.

Contact Us

If you need expert legal representation for a DWI case, don't hesitate to contact Zendeh Del & Associates, PLLC. We are here to help you navigate the complexities of DWI defense and protect your rights.

Zendeh Del & Associates, PLLC

By staying informed about the limitations of SFSTs in detecting marijuana impairment, we can better protect the rights of those accused of DWI and ensure a fair legal process