Marijuana does not reliably cause horizontal gaze nystagmus (HGN), the involuntary jerking of the eyes that police officers check for during roadside sobriety testing. Traditional law enforcement training has long classified HGN as a hallmark of alcohol and certain sedative drugs, not cannabis. Yet the science is not a clean “never.” A handful of studies have recorded HGN in some marijuana users under specific conditions, and a 2024 systematic review found the evidence surprisingly inconsistent across different study designs. Understanding where the research actually lands matters, because officers, courts, and drivers routinely treat the HGN test as though it settles the question of what substance a person has consumed.
What Officers Are Trained to Expect
The Standardized Field Sobriety Test (SFST) battery used across the United States includes the HGN test as one of its three core components. An officer slowly moves a stimulus, usually a pen or fingertip, across the subject’s field of vision and watches for three clues in each eye: the eye cannot follow the stimulus smoothly, it jerks distinctly at maximum deviation, and the jerking starts before the eye reaches a 45-degree angle. The Drug Recognition Expert (DRE) protocol, a more advanced evaluation, further categorizes which substances are likely responsible for specific eye signs. Under DRE training, HGN is expected from central nervous system depressants like alcohol, barbiturates, and benzodiazepines, as well as certain inhalants and dissociative anesthetics. Cannabis is not on that list.
This training distinction was part of the rationale when a 2023 randomized clinical trial at the University of California, San Diego chose not to test HGN in cannabis-dosed participants at all. The researchers cited DRE input that HGN “is unlikely to be affected by THC” and dropped it from the protocol to save time for other assessments.1PubMed Central. Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial That decision reflects how firmly the assumption is embedded in current practice. But assumptions and evidence are not always the same thing.
What Controlled Studies Have Found
The most frequently cited laboratory evidence against a cannabis-HGN link comes from an older study in which 72 volunteers smoked marijuana and were then assessed for nystagmus using electronystagmography, a sensitive recording technique. No nystagmus was detected.2Journal of Forensic and Legal Medicine. Toxicology observation: Nystagmus after marijuana use That 72-subject finding carried a lot of weight for years, in part because it aligned with DRE training and in part because few researchers bothered to revisit the question.
When researchers did revisit it, the results were not as tidy. A placebo-controlled study involving heavy cannabis users found that acute THC inhalation increased the proportion of participants showing HGN impairment from zero at baseline to about 15 percent, but the change only approached statistical significance and did not cross the conventional threshold.3PubMed Central. A placebo-controlled study to assess Standardized Field Sobriety Tests performance during alcohol and cannabis intoxication in heavy cannabis users and accuracy of point of collection testing devices for detecting THC in oral fluid That 15 percent figure is not nothing, but it is far lower than the rates typically seen with alcohol, where HGN appears in the vast majority of intoxicated individuals. The researchers concluded that impairments on the HGN test in combined alcohol-and-cannabis conditions were “most likely attributable to alcohol,” since many participants in the study had developed tolerance to THC’s impairing effects.
A 2024 systematic review of oculomotor deficits associated with cannabis pulled together evidence from seven studies that evaluated nystagmus specifically. The results were all over the map. In DRE evaluations of drivers suspected of impaired driving who tested positive for THC alone, no HGN impairment was observed compared to drivers who were not positive for alcohol or other drugs. But two randomized controlled trials did show statistically significant increases in HGN after participants smoked THC-containing cigarettes, measured at time points ranging from five minutes to nearly two hours after administration. Another study found a trend toward increased HGN after acute inhalation, without reaching full statistical significance.4PubMed Central. A systematic review of oculomotor deficits associated with acute and chronic cannabis use So the answer the research gives is not a clean “no.” It is closer to “usually not, but sometimes yes, and we cannot fully explain the inconsistency.”
Chronic Use Adds a Twist
One of the more unexpected findings in that same systematic review involved people with cannabis use disorder. Participants with ongoing heavy cannabis use who met diagnostic criteria for the disorder exhibited a significant increase in HGN impairment during field sobriety testing compared to matched drug-free controls.5PubMed Central. A systematic review of oculomotor deficits associated with acute and chronic cannabis use – Section: 3.5.3 Nystagmus This is a different scenario from a single smoking session. It suggests that long-term, heavy cannabis use may alter oculomotor function in ways that show up on a test designed to catch alcohol impairment.
The mechanism behind this is not settled. One possibility is that chronic cannabinoid exposure changes the sensitivity or calibration of the brainstem circuits that control smooth-pursuit eye movements. Another is that heavy users who show HGN are also using other substances that go undetected, though the study tried to control for that. A third possibility is simply that chronic impairment of the cerebellum, which helps coordinate eye movements, accumulates over years of heavy use. Whatever the explanation, this finding complicates the tidy story. If you are a daily, heavy cannabis user, your eyes might behave differently during an HGN test even if you have not smoked in the last few hours.
Why Cannabis Doesn’t Trip the Same Neural Switches as Alcohol
The reason HGN is such a reliable marker for alcohol and sedatives but not for cannabis comes down to where these substances act in the brain. Alcohol and benzodiazepines suppress activity broadly across the central nervous system, including the brainstem nuclei and cerebellar pathways responsible for stabilizing the eyes during tracking. When these circuits are depressed, the eyes cannot follow a moving target smoothly, and the characteristic jerking motion appears.
Cannabis works through the endocannabinoid system, primarily by activating CB1 receptors. These receptors are abundant in the cortex, hippocampus, and basal ganglia, but they are less densely concentrated in the brainstem regions most directly responsible for generating the type of eye movements measured by the HGN test. The systematic review noted that oculomotor processes originating in cortical regions with low CB1 receptor density tend to be less affected by cannabis.4PubMed Central. A systematic review of oculomotor deficits associated with acute and chronic cannabis use Cannabis-related deficits in eye tracking, when they appear, are more likely to arise from disruptions further downstream in neural processing rather than from a direct hit to the gaze-stabilization machinery.
This distinction explains the pattern in the research. Cannabis does affect the eyes, sometimes quite visibly, but the effects tend to show up in different tests and in different ways than alcohol does. The HGN test was designed to catch alcohol, and it catches alcohol well. Expecting it to perform the same way for cannabis is like using a metal detector to find plastic.
A Rare but Real Case Report
There is at least one documented clinical case that complicates the dominant narrative further. A case report described an adult male who presented with three days of visual disturbance and dizziness following marijuana use. His examination was notable for gaze-evoked nystagmus and ataxia, and his lab work was unremarkable except for a urine drug screen positive for marijuana only.2Journal of Forensic and Legal Medicine. Toxicology observation: Nystagmus after marijuana use The authors explicitly noted that the finding ran counter to the traditional teaching that HGN is a sign of sedative intoxication, not marijuana.
A single case report cannot overturn the weight of controlled studies, and case reports come with obvious limitations: you cannot confirm causation, you cannot rule out every possible confounder, and you do not know how the patient’s individual neurology may differ from the norm. Still, the case is worth knowing about because it demonstrates that cannabis-associated nystagmus can occur in specific individuals, even if it is not the expected or typical response. If you are an attorney, a police officer, or a driver, knowing that the rule has documented exceptions matters.
What Happens When Cannabis and Alcohol Mix
The picture changes substantially when alcohol enters the equation, which is relevant because many impaired drivers have consumed more than one substance. In the same placebo-controlled study that found THC alone produced only a borderline increase in HGN, the combination of alcohol and cannabis produced a statistically significant relationship with HGN impairment. The percentage of individuals showing impairment increased in a dose-dependent manner when both substances were on board.3PubMed Central. A placebo-controlled study to assess Standardized Field Sobriety Tests performance during alcohol and cannabis intoxication in heavy cannabis users and accuracy of point of collection testing devices for detecting THC in oral fluid
The researchers were careful to point out that the HGN impairment in the combined condition was most likely driven by the alcohol component. This makes pharmacological sense given the receptor-density explanation above: alcohol hits the brainstem circuits that produce HGN, and THC largely does not. But from a practical standpoint, a person who has used both substances and then fails the HGN test during a traffic stop may be identified as alcohol-impaired even if the cannabis was the more recent or subjectively impairing substance. The HGN test cannot tell the officer which drug is responsible for the result.
What Cannabis Actually Does to Your Eyes
If HGN is not a reliable marker for cannabis, what eye signs are? The DRE protocol does include ocular indicators for cannabis, but they are different ones. Lack of convergence, the inability to cross your eyes when focusing on an object moved slowly toward the bridge of your nose, is one of the signs DRE officers look for in cannabis suspects. Pupil dilation in certain lighting conditions is another. Bloodshot, watery eyes (conjunctival injection) are a third, though this sign is so nonspecific that it can be caused by allergies, fatigue, dry air, or contact lens wear.
Research into pupil response as a biomarker for cannabis use has been gaining traction. A 2024 study investigating pupil response to light as a potential digital biomarker for recent cannabis use noted that while some oculomotor measures like saccadic accuracy and fixation duration show consistent effects across studies, “vertical and horizontal nystagmus are not reliably reproduced or depend on the administered test.”6PubMed Central. A Study of Pupil Response to Light as a Digital Biomarker of Recent Cannabis Use In other words, the inconsistency of nystagmus findings is itself a consistent finding, and researchers are looking elsewhere for better indicators.
Technology-assisted eye tracking, using infrared cameras to measure pupil dynamics and fixation patterns, shows more promise than the pen-and-flashlight approach of the traditional SFST. These tools can detect subtler changes in how the eyes behave that a roadside officer simply cannot see with the naked eye. But none of these technologies are standard issue for patrol officers yet, and their legal admissibility is still working its way through the courts.
Why the Broader SFST Battery Struggles with Cannabis
The unreliability of HGN for cannabis is part of a larger problem. The entire SFST battery was developed and validated for alcohol impairment, and it performs poorly when applied to cannabis alone. A study funded by the National Institute of Justice found that the one-leg stand, walk-and-turn, and modified Romberg balance tests were not sensitive to cannabis intoxication for any of the study participants.7National Institute of Justice. Field Sobriety Tests and THC Levels Unreliable Indicators of Marijuana Intoxication This does not mean cannabis is safe to drive on. It means the tools currently used to detect it at the roadside were not designed for the job and do not perform well at it.
Cannabis impairment manifests differently from alcohol impairment. Cannabis tends to affect attention, time perception, and decision-making more than gross motor coordination. A person who is meaningfully impaired by THC might walk a straight line just fine but fail to notice a pedestrian entering a crosswalk. The SFST battery, including the HGN test, measures the kinds of impairment alcohol produces, not the kinds cannabis produces. This mismatch has real consequences for both enforcement and for individual rights. Drivers impaired by cannabis may pass tests that should flag them, while drivers who are not impaired may fail components for unrelated reasons.
The Edibles and High-Potency Question
Most of the controlled research on cannabis and HGN was conducted using smoked marijuana at relatively modest THC concentrations, often in the range of 2 to 4 percent THC. Today’s cannabis products frequently exceed 20 percent THC for flower and can reach 80 to 90 percent for concentrates. Edibles introduce an entirely different pharmacokinetic profile: slower onset, longer duration, and metabolism through the liver that produces 11-hydroxy-THC, a metabolite that crosses the blood-brain barrier more efficiently than THC itself.
Whether these higher-potency or differently delivered forms of cannabis might produce HGN more reliably than the low-dose smoked product used in older studies is genuinely unknown. The research has not caught up. It is plausible that very high doses of THC could produce enough generalized neural disruption to affect brainstem oculomotor circuits, but that remains speculation. The case report of gaze-evoked nystagmus lasting three days after marijuana use hints that unusual exposures can produce unusual results, but one anecdote does not make a pattern. Until controlled studies test modern-potency products, the honest answer is that we are generalizing from an era of weaker cannabis to an era of much stronger cannabis, and the generalization may not hold perfectly.
What This Means If You Are Pulled Over
If an officer administers the HGN test during a traffic stop and finds no nystagmus, that result is consistent with cannabis impairment but does not confirm it. If the officer does find HGN and you have only consumed cannabis, the nystagmus is more likely caused by something else: a medical condition, medication, fatigue, or the rare individual variation that the case literature documents. Officers trained in the full DRE protocol know this and are supposed to rely on a broader constellation of signs rather than any single test. But not every officer on a routine patrol stop has DRE training, and the HGN test is part of the standard battery administered to every suspected impaired driver.
For legal purposes, the absence of HGN in a cannabis-only case is actually expected and should not count against you. Conversely, the presence of HGN in someone who has only consumed cannabis should prompt a search for alternative explanations. Defense attorneys in marijuana DUI cases have increasingly pointed to the scientific evidence that THC does not reliably cause HGN, and some courts have begun to weigh this distinction. The science supports a straightforward position: HGN is a test for alcohol and certain depressant drugs, it was never designed to detect cannabis, and the evidence that cannabis alone produces it remains weak, inconsistent, and limited mostly to chronic heavy users or specific controlled conditions that do not map neatly onto roadside encounters.