Research that analyzed past studies on THC levels and impairment found that cannabis breathalyzers may never work.

Marijuana breathalyzers have been in development for years. These mythical tools could solve one of the main issues associated with the legalization of marijuana: measuring THC impairment in drivers. But a new study reveals that current breathalyzers are nowhere near that goal.

The study, conducted in Australia by researchers at the University of Sydney, found that marijuana breathalyzers were inconsistent in measuring impairment from THC. Researchers analyzed 28 studies on driving performance and concentrations of THC in blood and saliva and found the connection between the two inconsistent.

The idea of marijuana breathalyzers is based on alcohol breathalyzers, which are administered on the road and provide an accurate assessment of people’s blood alcohol levels. This has been efficient over the years in providing a relatively accurate take on people’s intoxication levels and how it affects driving skills. This doesn’t appear to be the case with THC.

This new study analyzed a variety of older studies that focused on how THC affected people’s reaction time and divided attention, skills that are necessary for driving safely. While the study found some strong connections between THC levels and impairment in inexperienced cannabis users, once cannabis users were seasoned (using the drug several times a week), these connections disappeared.

“Higher blood THC concentrations were only weakly associated with increased impairment in occasional cannabis users while no significant relationship was detected in regular cannabis users,” said Dr. Danielle McCartney, lead author of the study.

“This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis-THC-induced impairment.”

While THC intoxication can impair people’s driving skills, it presents itself very differently depending on the person consuming THC. Someone who’s experienced with cannabis might show the same levels of THC in their blood as someone who’s inexperienced with it. These two people will likely have completely different responses to the drug and how impaired they are by it.

Marijuana breathalyzers were people’s go-to response for solving driving while under the influence of THC. Now, it appears that these devices should measure a different biomarker for success, something that proves not only that someone consumed THC recently, but that they’re impaired by it. While the idea of a device that can measure THC sounds safe enough for providing an accurate assessment of intoxication and for discouraging this type of behavior, marijuana is too complex a drug to be reduced by a number.