In some states, if you are pulled over for a suspected DUI and do not submit to a breathalyzer test, you may be detained and given a mandatory blood test. Blood tests are typically much more accurate at detecting blood alcohol content, but what about other drugs, like marijuana?
Will marijuana show up on a blood test?
Typically, law enforcement officials will have to run a specific test to detect Tetrahydrocannabinol (THC), the active ingredient in marijuana, in your blood. In many cases, unless you are suspected of having consumed marijuana because of prior evidence, the police will not run the test.
It's also likely that if there is enough alcohol in your system to charge you formally with DUI, there is no need to test for marijuana.
Despite these realities expressed by some DUI lawyers and forensic analysts, police can always opt to run a full toxicology test, and if you have consumed marijuana, there is always the possibility that it will be found in your blood.
What does marijuana do in the body?
The way that humans break down THC is complex and often problematic for prosecutors who charge people with driving under the influence of marijuana.
First, a blood test can really only detect a significant amount of marijuana if you smoke the drug with some level of frequency or if you smoked it within maybe an hour of being tested. THC levels in the blood go up very quickly when you first smoke, but recede just as fast, and blood tests often make it difficult to determine how much was smoked, when, and if its effects affected your ability to drive. It's possible, for instance, to test positive for marijuana via second-hand ingestion that you were completely unaware of.
Certain common medications, including over-the-counter pain drugs, can create a false positive.
What about the legal ramifications?
In some states, the legal end of marijuana testing is complicated and often more difficult to enforce than a DUI as a result of alcohol. In California, the Department of Motor Vehicles can immediately suspend a license following a DUI arrest and positive blood test, but cannot do so solely for the detection of marijuana in the blood.
Law enforcement officials are often much more experienced with assessing the symptoms of alcohol consumption than they are the consumption of illegal drugs, and even if they correctly suspect intoxication, their lack of expertise may be a liability for the prosecution in court.
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