The legalization of marijuana in the US remains a hot-button issue, despite ongoing progress toward both medical and recreational legalization in numerous US states. There are those who believe that people should have the right to smoke marijuana if they choose, while others see it as a public health concern.

Opponents insist that there are many reasons that recreational use of Cannabis sativa and Cannabis indica should remain illegal, mainly due to the belief that cannabis use is a gateway drug that could lead to future experimentation with “harder” drugs that have more pronounced safety risks and addiction potential. But, despite the years of anti-drug PSAs and “gateway drug” rhetoric, the science is just not there to make those type of assertions. In fact, many of the typical arguments against marijuana legalization don’t necessarily hold up to scientific scrutiny, implying that it would be ill-advised to allow such assumptions to influence decisions about legalization or criminalization.

Today, several US states have legalized recreational marijuana use. This has brought up the issue of driving while intoxicated. When you drink alcohol, there is a clear definition in place to define who is intoxicated, and who is not, and how to proceed with prosecution. Field sobriety tests, breathalyzers, blood tests, and other means can prove that someone is acutely intoxicated with ethanol. Studies in Baltimore DUI cases have been done for decades to determine at what point someone is impaired after drinking alcohol, and a blood alcohol content of 0.08 is widely accepted as the standard threshold at which a typical person with a normal alcohol tolerance can be reasonably assumed to be impaired by alcohol’s effects.

Although each person is different, the standard threshold of a BAC of 0.08 is based on decades of research and careful consideration by both legislators and the public.  There is reasonable confidence in the statement that when a person’s blood alcohol content reaches this threshold, their ability to drive is impaired.

Being under the influence of marijuana is different. There are no clear boundaries set for when someone is impaired while under the influence. The science, in its infancy, is simply not there to make determinations about how to set standards, test, or to be certain that someone is under the influence and a danger when behind the wheel. While alcohol intoxication is something that you can test for, on the spot, there’s no established way of verifying, for sure, that a person is under the effects of cannabis.

As states like California and Colorado have legalized recreational marijuana use, there are still many questions about the active ingredient, THC (delta-9-tetrahydrocannabinol), and how it may or may not impair someone’s ability to drive or operate a motor vehicle.

Government agencies have determined a threshold for THC in the blood, as a guide to determine who is intoxicated and who is not. But, the truth is that there is just no scientific data that those boundaries are based on anything concrete.

Without much time to distinguish how the body reacts to THC both physically and mentally, much more time is needed to research marijuana effects before it can legislated. The problem is that there simply isn’t enough time for law enforcement to wait. Without anything to guide them, making a case against someone who may be endangering themselves or the public remains all but impossible. At the same time, it can be reasonably assumed that the effects of cannabis on cognition and perception probably aren’t conducive to good driving.

The body processes alcohol very differently from the way that it does THC. Alcohol is a substance that is metabolized by the body at a quicker rate after ingestion. The problem with THC is that it doesn't metabolize quickly. Therefore, you may still have THC in your blood long after you have ingested it. That may put people at a disadvantage when it comes to using blood to test for impairment and conviction. Someone who smoked or ingested marijuana three days ago may still have detectable metabolites in their body, but may not feel the effects at all.

THC may be present in the body long after it has any effect on your motor or mental skills. That means that even if you are tested for THC, the results may say nothing about your impairment at the time of testing. It suggests that you have used marijuana in the relatively recent past, but has no relation to the here and now. Because detectable cannabinoid metabolites are present in the body long after the substance’s effects have subsided, it’s difficult to use blood testing to determine whether a person is currently intoxicated, or was intoxicated at a particular time.

The second problem is that every body metabolizes pot differently. Some people will have a high concentration in their blood well after ingesting it, and will show no effect, while others may have very little and may be highly intoxicated. That negates any real or standard way to test effectively for intoxication time-specifically. Blood, hair, or urine tests cannot reliably determine the recency, frequency, or amount of use.

Until the science catches up, using THC levels to indicate a person’s level of impairment is simply a guessing game, and has no basis on anything real or concrete. The best way to test for impairment is a field sobriety test, but law enforcement may have their hands tied. A more subjective way to prove guilt, the convictions don’t hold up as well in court when challenged.

As more states legalize marijuana, convicting people of DUIs for marijuana use will remain a shaky science at best. Only time will tell if some real and scientific thresholds can be found, but the reality is that testing someone’s blood simply isn’t going to cut it. This is a matter of concern, because the known effects of cannabis do include things that could potentially impair a person’s ability to drive normally.


Published by Peter Garlow