Driving While Intoxicated (DWI) offenses are typically associated with alcohol. However, it is important to remember that DWI pertains to alcohol as well as other drugs. Section 49.01 of the Texas Penal Code defines intoxication as, “Not having the normal use of mental or physical faculties by reason of the introduction of alcohol, a controlled substance, a drug, a dangerous drug, a combination of two or more of those substances, or any other substance in the body.”

With the widespread legalization and acceptance of marijuana, police are encountering stoned drivers almost as often as drunk drivers. So, what is the difference between the two? It turns out, a lot.

Is Stoned Driving Worse than Drunk Driving?

There are various studies that show driving stoned can be just as deadly as drunk driving. The problem, however, is that intoxication by marijuana is highly variable. With alcohol, there is a considerable body of research that can associate risks with increasing BAC levels. But, with marijuana, a more precise association of THC levels and degrees of impairment is not yet available.

The Science Behind Marijuana Intoxication

The effects of various drugs differ greatly, including both alcohol and marijuana. Many people understand the harm caused by alcohol-impaired driving because the pharmacokinetics (absorption, distribution, and elimination of a drug from the body) and pharmacodynamics (how a drug affects our body and behavior) have been widely studied. Yet these processes differ substantially for other drugs, including marijuana.

With alcohol, impairment increases with rising alcohol concentration and declines with dropping alcohol concentration. This well-established relationship is the basis for laws prohibiting driving while intoxicated.

In comparison, the absorption, distribution, and elimination rates for marijuana as well as behavioral and cognitive effects are very different than with alcohol The primary psychoactive substance in marijuana is delta-9-tetrahydrocannabinol (THC). Whereas alcohol is water-soluble, THC is fat-soluble. Once ingested, THC is stored in fatty tissues in the body and can be released into the blood long after ingestion. Some studies have detected THC in the blood 30 days after ingestion. Even though marijuana can be detected in the blood long after ingestion, the psychoactive effects of marijuana only last for a few hours—not days or weeks.

Thus, the problem becomes the inability to determine the occurrence of THC’s intoxicating effects. For example, let’s say you ingested marijuana at 6 pm. If at 10 pm you were pulled over and subsequently arrested for DWI, your blood results would reflect active THC in your system. However, THCs presence alone does not mean you were actually intoxicated. It is likely you felt the intoxicating effects of the THC 20 to 30 minutes after ingestion—not 4 hours later.

Per Se Problems

While all 50 states have adopted a per se alcohol limit of .08, not all states have adopted a similar standard for marijuana.

Some states have set a THC limit in their laws indicating that if a suspect’s THC concentration is above that level (typically 5 ng/mL of blood), then the suspect is considered impaired. Unlike alcohol per se limits, these limits for marijuana are not based on scientific evidence.

The AAA Foundation for Traffic Safety published a study in 2016 concluding there was no clear evidence that 5 ng/mL is a good determinator of impairment. One consideration that undermines the effectiveness of a per se standard for THC is that the person using marijuana has no meaningful way of knowing their blood THC concentration.

Even though Texas has not adopted a per se THC limit, if you have any amount of THC in your system and have lost the normal use of your mental or physical faculties, you can be charged with DWI.

 SFSTs and Marijuana

If you are pulled over and suspected of driving while intoxicated—either by alcohol or marijuana—there is a good chance the officer will ask you to perform Standard Field Sobriety Tests (SFSTs). These tests were originally designed to correlate the subject’s performance on the FST’s with a BAC. In Texas, officers mainly use SFSTs to determine whether or not someone has lost their mental and physical faculties due to a substance.

Because THC is much different than alcohol, studies have shown that the SFSTs are completely unreliable when it comes to detecting marijuana impairment. In 2017, the Massachusetts Supreme Court found that the SFSTs are not “scientific tests” for THC impairment.

We are not persuaded, however, that the FSTs can be treated as scientific tests establishing impairment as a result of marijuana consumption. The scientific community has not reached a consensus as to whether a defendant’s performance on any combination of FSTs, or on any individual FST, is correlated with marijuana use or impairment.

Massachussetts Supreme Court

If you are charged with a marijuana DWI, make sure to contact a criminal defense lawyer who can protect your rights and aggressively fight your case.

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