Blood tests used for driving under the influence (DUI) of marijuana cases can detect marijuana over 12 hours after ingestion. For frequent users, the drug can stay in your blood for up to a week.
This evidence can be used against you in a drugged driving case even though you were not impaired by marijuana at the time of your arrest.
Can blood tests detect THC?
Yes, blood tests can detect the presence of tetrahydrocannabinol (THC), the psychoactive substance in marijuana.
Blood tests measure cannabinoids or cannabinoid metabolites in a blood sample. Marijuana has over 100 different cannabinoids. These cannabinoid metabolites are created when your body breaks down marijuana.
Typically, blood tests will measure the levels of delta-9-THC or its metabolites in the blood sample. Less often, blood tests will measure the levels of cannabidiol (CBD) in the sample.
Are they reliable for marijuana DUI cases?
Blood tests do not provide reliable evidence of marijuana-impaired driving for several reasons:
- THC stays in your blood for long after its psychoactive effects have worn off,
- most states do not have an established legal limit for THC,
- different people get impaired by different THC levels,
- how often you use marijuana increases how long it stays in your bloodstream,
- because THC is stored in body fat, the more body fat you have, the longer it will last in your blood, and
- how you ingested marijuana will alter how long it stays in your bloodstream.
Most of these reasons have to do with timing. Numerous factors alter how long THC stays in your bloodstream. Many of them have nothing to do with how impaired you were at the time of the traffic stop. A blood test can detect marijuana that you smoked or ingested up to a week before. It can be several days since you were too impaired to drive.
Nevertheless, the blood test can be used as evidence that you were too impaired by marijuana to drive safely.
Can other tests detect cannabis?
There are several tests that can detect THC, including:
- urine tests,
- saliva tests, and
- hair samples.
While they may be more feasible to use in the field, many of these tests have the same timing problems as blood testing.
Urine testing can detect marijuana that was ingested by a casual user up to 3 days ago. Heavy users can have detectable levels of marijuana in their urine sample for 10 days or more.[1]
Saliva testing can detect THC in oral liquids. However, saliva testing can produce positive results from a swab for up to 29 days after ingestion.[2]
Hair samples can also be used to detect marijuana, as well. However, hair samples are useless for DUI cases because it takes around a week for THC, CBD, or cannabinol (CBN) to begin to appear in hair follicles. Once the chemicals are stored in hair follicles, they can remain there for several months.[3]
Currently, there is no reliable breath test for marijuana. However, scientists are working on one that detects the presence of marijuana in your breath.[4]
As a result, THC blood tests are still regarded as the best for DUIs involving marijuana, even though they cannot be performed during the traffic stop.
What other evidence can be used in a drugged driving case?
While law enforcement officers will generally use the results of a blood test in a DUI-drugs case, they often rely on other pieces of evidence, as well. Some types of evidence that can be used against you in a marijuana DUI case include:
- the arresting officer’s testimony that he or she smelled marijuana in the vehicle,
- there was marijuana in the car, which can also lead to charges of drug possession in states without legalization,
- the presence of drug paraphernalia in the vehicle, which can lead to additional charges of possession of drug paraphernalia, and
- the arresting officer saw signs of impairment, but the chemical test results did not show any alcohol in the person’s system.
In many drugged driving cases, there is no blood test. The case will rely entirely on these other types of evidence.
Testimony from the arresting officer is especially common in drugged driving cases. It often comes in the form of generalized descriptions of behavior that are typically associated with marijuana use, such as:
- general anxiety or an unnatural calmness,
- bloodshot or red eyes,
- poor reaction times,
- laughter,
- restlessness, and
- inattentiveness or frequent distraction.
Without footage from the officer’s body camera or dashboard camera, it can be difficult to refute this testimony. In criminal DUI cases, juries generally believe the police officer over the defendant.
What is the legal limit for marijuana?
There is no generally accepted legal limit for THC or marijuana. This makes drugged driving cases very different from drunk driving cases.
Laws that prohibit driving under the influence of alcohol use a legal limit. If you are found to have a blood alcohol content (BAC) at or above that limit, you are per se under the influence. That legal limit is generally 0.08 percent. However, there are some exceptions, like for:
- drivers under the age of 21, whose legal limit is 0.02 percent in some states,[5] and 0.00 percent in others,[6]
- commercial drivers, whose legal limit is generally 0.04 percent,[7] and
- the state of Utah, where the legal limit for normal drivers is now 0.05 percent.[8]
If your BAC level is at or above the applicable legal limit, you are too impaired to drive by law. It is extremely difficult to prove that you were not impaired if a chemical BAC test found you to be over the legal limit. These drunk driving cases are often resolved quickly.
However, very few states have adopted a legal limit for blood THC content. Those that have adopted a legal limit have generally set it at 5 nanograms per milliliter (ng/ml) of blood.[9] This particular choice of a legal limit, however, has been criticized as an arbitrary level of THC concentration.[10] Even these states, though, disagree on what this means:
- Washington state uses this as a “hard” legal limit much like for drunk driving, where being over the limit means you are per se impaired,[11] while
- Colorado treats it as a “soft” legal limit, where being over it creates a permissible inference that you were under the influence of marijuana.[12]
By hiring a criminal defense attorney from a reputable law firm to fight against a DUI charge, you can protect your rights and future.
For how long does marijuana impair driving?
The effects of marijuana are subjective. Some people get more impaired than others on similar amounts of THC. Additionally, how you ingest marijuana can change how long you feel its effects. However, one study found that most people stopped feeling impaired 3 hours after smoking marijuana.[13]
What is the law in California?
California prohibits driving a vehicle while under the influence of any drug.[14] This includes marijuana. However, there is no per se legal limit for THC in your blood. Instead, prosecutors have to show that:
- you drove a vehicle, and
- while driving, you were under the influence of a drug.[15]
This requires proof that your mental or physical abilities were so impaired that you could no longer drive safely. Poor driving is a factor in making this determination, but cannot be the only one considered. All of the surrounding circumstance have to be taken into account.[16]
Because so many different factors can be considered, having a skilled DUI attorney on your side is essential. They can help you raise an effective DUI defense to combat the evidence against you, such as a lack of probable cause to initiate the traffic stop.
Legal References:
[1] Moeller K, et al., “Clinical Interpretation of Urine Drug Tests,” Mayo Clinic Proceeding, 92(5):774-796 (May 2017).
[2] Lee D and Huestis M, “Current Knowledge on Cannabinoids in Oral Fluid,” Drug Testing and Analysis, 6(0):88-111 (Aug. 25, 2013).
[3] Taylor M, et al., “Comparison of Cannabinoids in Hair with Self-Reported Cannabis Consumption in Heavy, Light, and Non-Cannabis Users,” Drug and Alcohol Review, 36(2):220-226 (March 2017).
[4] Denise Chow, “Scientists Say a Marijuana Breathalyzer is in the Works,” NBC News (Oct. 6, 2022).
[5] Georgia Code Annotated 40-6-391(k)(1).
[6] Maine Revised Statute Title 29-A, Section 2411.
[7] See, e.g., California Vehicle Code 23152(d) VEH.
[8] Utah Code Annotated 41-6a-502.
[9] See Colorado Revised Statute 42-4-1301(6)(a)(IV), and Revised Code of Washington 46.61.502(1)(b). But see Nevada Revised Statute 484C.110(3) (setting the legal limit at 2 ng/ml for delta-9-THC but 5 ng/ml for 11-OH-THC).
[10] Kaitlin Sullivan, “‘Arbitrary’ Cutoffs of THC Levels Make it Difficult to Measure Impaired Driving,” NBC News (Jan. 12, 2022).
[11] Revised Code of Washington 46.61.502(1)(b).
[12] CRS 42-4-1301(6)(a)(IV).
[13] DeGregorio M, Wurz G, Montoya E, and Kao C, “A Comprehensive Breath Test That Confirms Recent Use of Inhaled Cannabis Within the Impairment Window,” Scientific Reports, 11:22776 (Nov. 23, 2021).
[14] California Vehicle Code 23152(f) VEH.
[15] California Criminal Jury Instructions (CALCRIM) No. 2110.
[16] Same.
