California’s DUI laws can be complex and confusing. In this section, our attorneys break down the rules and explain the process.
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There is no universally accepted breath test for suspected cases of driving under the influence of marijuana. Police often request that you submit to a blood test, instead, or to field sobriety tests (FSTs). However, the results from these other tests are unreliable for determining marijuana impairment. The testimony of the officer who initiated the traffic stop may also be used against you.
While breath tests have been invented for marijuana DUIs, none of them has proven to be reliable.
These breath tests aim to detect delta-9-tetrahydrocannabinol (THC) on your breath. They are supposed to work like breath tests for alcohol, measuring blood alcohol content (BAC). However, no marijuana breath test has produced reliable results.
Police often turn to blood tests to detect marijuana impairment. These test your blood for THC.
After you ingest marijuana, THC, the active ingredient in the drug, attaches to your body’s fat cells. Those THC metabolites are gradually eliminated through the normal digestion process.
Blood tests can detect how much THC is in your bloodstream by measuring these metabolites.
Unfortunately, those metabolites are not eliminated at the same rate. Different people have different elimination rates. Some factors include:
As a result, THC can still be detected in your blood up to 12 hours after you have ingested it. This can be longer than you would be too impaired by the drug to drive. This can lead to false positives, where you are sober but you fail the blood test.
Because these tests require a blood draw, they cannot be conducted during the traffic stop. Police will generally have to detain you and bring you to a hospital or the police station for the blood draw. To make a DUI arrest, they must have probable cause to believe you are too impaired to drive a motor vehicle. Additionally, police may need a warrant to require you to submit to a blood test.
A DUI lawyer can invoke your rights and protect your interests.
Police can use 2 other types of test to detect THC in your body:
Both of these tests have some of the same drawbacks as a blood test.
Urine tests can detect THC in a person’s system up to 30 days after you ingested it. Additionally, they generally cannot be conducted during the traffic stop.
Saliva tests can be done during the traffic stop with a simple swab. However, they can detect THC in your saliva up to 24 hours after you ingested it.
The long detection windows of both of these test results can lead to false positives.
Law enforcement officers often use FSTs to detect cannabis impairment. However, studies have shown that they are very unreliable.
Most FSTs are designed to test your:
A couple of common FSTs that police use when they suspect that you are under the influence of marijuana are the:
However, these and other FSTs are poor indicators of marijuana impairment. One study involved 184 participants who were either given THC or a placebo. Police officers then conducted FSTs on them. The officers claimed that:
The arresting officer’s testimony plays a big part in DUI cases involving marijuana. Officers generally report that they:
Some police officers have special training in detecting signs of drug impairment. Known as drug recognition experts, or DREs, many have taken a course on drug detection. The requirements for DRE certification vary by state.
Since the legalization movement, a couple of states have passed laws that set a legal limit for marijuana impairment while driving. For example, in Montana, you are per se under the influence of marijuana, or presumed to be too impaired to drive, if a test shows that you have 5 nanograms of THC per milliliter of blood.
This is similar to the per se legal limit of 0.08 percent for BAC in cases involving the influence of alcohol.
However, there is no scientific consensus that this level of THC makes you impaired.
California does not have a per se legal limit for the amount of THC you can have in your blood.
The DUI defense lawyers at our law firm have found that police officers in the state generally use the following types of evidence in DUI marijuana cases:
They may occasionally use saliva or urine tests.
Getting an experienced criminal defense attorney is the best way to fight against a DUI charge for marijuana.
 Chris Roberts, “Study: Sure Looks Like ‘Marijuana Breathalyzers’ Don’t Work – And May Never Work,” Forbes (December 31, 2021).
 Supra note 2.
 National Institute of Justice, “Field Sobriety Tests and THC Levels Unreliable Indicators of Marijuana Intoxication,” (April 5, 2021).
 T Marcotte, A Umlauf, D Grelotti, “Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial,” JAMA Psychiatry 2023;80(9):914-923 (August 2, 2023).
 Montana Code Annotated 61-8-1002.
A former Los Angeles prosecutor, attorney Neil Shouse graduated with honors from UC Berkeley and Harvard Law School (and completed additional graduate studies at MIT). He has been featured on CNN, Good Morning America, Dr Phil, The Today Show and Court TV. Mr Shouse has been recognized by the National Trial Lawyers as one of the Top 100 Criminal and Top 100 Civil Attorneys.