Cannabis and Alcohol Together: Impairment and Field Signs
By THE OKOROCHA FIRM*
Cannabis and Alcohol Together: The Combination Matters, but the Field Signs Still Have Limits
Cannabis plus alcohol is not the same evidentiary problem as cannabis alone. The combined exposure may increase impairment. It does not make every roadside observation scientifically specific to THC.
A 2010 study in Addiction by Bramness, Khiabani, and Morland examined suspected impaired drivers in Norway. The authors compared cases involving THC alone, THC with ethanol, ethanol alone, and no detected drug. They concluded that the combined effect was additive. At the same time, they reported that most of the 27 clinical tests and observations in the Norwegian clinical test for impairment had no relationship to blood THC concentration.
Both findings belong in the analysis. Ignoring the combination understates the evidence. Treating nonspecific field signs as proof of THC impairment overstates it.
The study population
The researchers used a retrospective forensic database of drivers apprehended by police on suspicion of impaired driving. Their analysis included 589 THC-only cases, 894 THC-plus-ethanol cases, 3,480 ethanol-only cases, and 79 cases in which no drug was detected.
After blood collection, a physician performed the Norwegian clinical test for impairment. The examination included 27 observations addressing alertness, cognition, balance, coordination, eyes, pulse, and general appearance. The physician also made an overall judgment about impairment.
This was not a randomized dosing experiment. It was an observational study of people selected because police already suspected impairment. That design gave the authors access to real-world cases and higher THC concentrations that may be difficult to study experimentally. It also created selection and comparison problems that limit causal interpretation.
What the combination showed
The study reported a marked increase in the proportion judged impaired when THC and ethanol were present together. The authors found no evidence that the joint effect was more than additive in their data. In other words, the results supported an additive effect, not proof of a synergistic effect greater than the sum of the individual effects.
That distinction matters. “More impairment with both” and “scientific proof of synergy” are not interchangeable claims.
The paper also reported increased impairment judgments when relatively low alcohol concentrations accompanied THC. A complete review should therefore account for alcohol rather than attempting to interpret the THC result in isolation.
For related discussions, see Drug Blood Testing and Crash-Risk Analysis and Drug Toxicology and Driving Impairment.
Most clinical observations did not track THC concentration
The same study found no relationship between blood THC concentration and most individual components of the clinical examination. The principal exceptions involved conjunctival injection, pupil dilation, pupil reaction to light, and the overall risk of being judged impaired.
Even those exceptions were not universal identifiers. The authors reported that dilated pupils appeared in no more than 35 percent of suspects even in the highest concentration ranges. They also acknowledged conflicting literature on whether cannabis causes pupil dilation, pupil constriction, or no change.
Red eyes and pupil findings are observations. They are not chemical identification tests, dose measurements, or exclusive markers of cannabis impairment. Fatigue, illness, lighting, medications, anxiety, and examination conditions may affect the same observations.
The study itself concluded that the Norwegian clinical examination did not offer sensitive tests or observations that reliably revealed THC impairment. That conclusion is especially important because the examination included more observations than the standardized field sobriety tests ordinarily used in the United States.
See Expert Witness Analysis of Field Sobriety Tests and Drug Recognition Expert Program Analysis.
Blood THC concentration still requires context
The authors reported an association between increasing blood THC concentration and the overall risk of being judged impaired in their selected population. That group-level relationship does not transform one blood concentration into an individualized impairment meter.
THC concentration changes rapidly after use, while effects and tolerance vary. The interval between driving, examination, and blood collection matters. The specimen matrix and analytical method matter. Chronic use can complicate the relationship between detected THC and the timing of acute exposure.
A scientifically responsible opinion should separate:
- Proof that THC was present in the blood specimen
- The concentration at the time of collection
- The likely concentration at the time of driving
- The person’s actual functional performance
- The contribution of alcohol or another substance
- The ultimate legal definition of impairment
Those questions interact, but no single observation answers all of them.
See THC Blood Levels Do Not Prove Impairment and Positive Drug Test Versus Impairment.
Study limitations that belong in the courtroom
The paper’s authors identified several limitations. The study was observational and exposed to selection bias. The drug-negative comparison group may have differed fundamentally from the drug-positive drivers. The ethanol-only group may not have been perfectly comparable to the THC-positive group.
The clinical examinations were conducted by numerous physicians under varying conditions. The doctors had different experience and motivation, no suspect was examined independently by more than one physician, and examination reliability was not checked afterward.
These limitations do not erase the study. They define what it can prove. It supports the proposition that combined THC and alcohol evidence deserves attention and that the observed effect in this population was additive. It also supports the proposition that common clinical signs were not reliable stand-alone detectors of THC impairment.
Questions for case review
When both substances are reported, counsel should obtain and compare:
- Exact driving, stop, examination, and blood-draw times
- The complete field notes and any video
- The blood-alcohol and cannabinoid analytical records
- Specimen type and collection history
- The laboratory’s identification and quantitation criteria
- Statements about drinking and cannabis use
- Medical, vision, fatigue, and medication evidence
- Evidence of tolerance and use history
- Whether the opinion concerns presence, recent use, impairment, or crash causation
The combination should be evaluated as a combination. But each inference still requires its own foundation.
The bottom line
The 2010 study reported additive impairment when THC and ethanol were detected together in a population of apprehended Norwegian drivers. It did not show that every THC-positive driver was impaired, that any particular concentration proved impairment, or that common roadside observations reliably identified cannabis influence.
The correct approach is neither dismissal nor exaggeration. Account for the combined substances, preserve the timing and population limits, and demand a specific scientific bridge from the evidence in the case to the opinion being offered.
Source
Jorgen G. Bramness, Hassan Zare Khiabani, and Jorg Morland, “Impairment due to cannabis and ethanol: clinical signs and additive effects,” Addiction 105(6) (2010) 1080-1087. PubMed record. DOI: 10.1111/j.1360-0443.2010.02911.x.
