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Driving Under the Influence of Marijuana

Posted by Ed McClees | Apr 20, 2018

When most people hear “DWI” they think about driving under the influence of alcohol.  But with an ever-increasing number of states legalizing either the medical or recreational use of marijuana, law enforcement has started to increase its attention to those who drive impaired due to their use of marijuana and other forms of tetrahydrocannabinol ("THC"), the active ingredient in marijuana. 

Texas DWI laws make it illegal to operate a motor vehicle in a public place while intoxicated.  Texas law defines intoxication as “not having the normal use of mental or physical faculties by 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 into the body.”  This definition is not limited to alcohol and includes someone who lost the normal use of his or her mental or physical faculties by the introduction of marijuana into that person's system.

Years ago the National Highway Traffic and Safety Administration introduced a series of standardized field sobriety tests to assist police officers in determining whether a driver was impaired.  The studies that formed the basis of these field sobriety tests focused on alcohol.  As more states have legalized recreational marijuana, there has been growing controversy as to whether traditional sobriety testing is appropriate for measuring the intoxication level of a driver who consumed marijuana, or some other form of THC.  

Police labs measure alcohol by having the accused citizen submit to either a breath test or a blood test.  The Intoxilyzer 9000, which is the machine used in Texas to detect alcohol upon a person's breath, is not capable of detecting THC upon a person's breath.  As such, if a person who consumed marijuana but consumed no alcohol provided a breath sample, that sample should read 0.00.  While police in some states have started experimenting with portable devices capable of detecting THC on drivers at the roadside, such devices are not yet used in Texas.

The presence of THC can, however, be detected by blood tests.  Many counties in Texas have started to focus on blood testing in their DWI investigations, and have obtained grants to fund “No Refusal” programs where police obtain search warrants to take blood samples from people who refuse breath tests.  Police can legally take a sample of a person's blood so long as they obtain the blood through either the accused's consent, or through a search warrant.  The blood sample is then analyzed to obtain a measurable amount of alcohol or other substance that may be in a person's blood.  

In addition to the “loss of the normal use of mental or physical faculties” standard discussed above, Texas law provides that a person is considered intoxicated if that person has a BAC of 0.08 or higher, regardless of whether that person has lost the normal use of his or her mental or physical faculties.   Some states have started to include a similar “per se” definition of intoxication regarding marijuana.  In Colorado and Washington, where recreational use of marijuana has been legalized, that limit is five nanograms per milliliter of blood, or five parts per billion. 

Texas has no “per se” definition of intoxication by the introduction of marijuana into the system, so courts and prosecutors look at this on a case-by-case basis.  There is still no common consensus as to what level of THC will consistently cause intoxication in most people.  It is important to note, however, that because recreational use of marijuana is not legal in Texas, many prosecutors and judges will often look harshly upon any detectable amount of THC that is found in the driver's blood. 

If you have been charged with an intoxication offense involving marijuana, call us.  Ed McClees is Board Certified in Criminal Law by the Texas Board of Legal Specialization, and has vast experience handling DWI cases of all types. 

About the Author

Ed McClees

Ed is a criminal trial lawyer who is Board Certified in Criminal Law by the Texas Board of Legal Specialization.  He is a former Harris County District Attorney where he served as chief of the Organized Crime Section, prosecuting cases involving complex organized criminal activities, including Fr...

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