What is the limit of THC you can have in your system before you are driving under the influence (DUI)?

"Prop. 64 defines driving impaired by marijuana to be a blood-THC level of 0.5 mg/ml. But blood-THC levels are irrelevant for determining impairment, according to the AAA, based on their experience in CO, WA, and OR. Only behavioral field tests can determine "drugged driving" now. Why should voters enshrine the irrelevant analytical standard of prop 64 in law?"

I am not sure why they will do this, but it does look like they will. Unfortunately, many are not willing to go through the law and read the individual pieces themselves.


As far as I can tell, there is NO definition in Prop 64 that defines how much THC you can have in your blood or found in your breath with a testing machine. There is $3 Million put aside in Prop 64 for the police to figure this out. If it is in the 64 pages of Prop 64, please copy and paste it so I may be better informed.

To prove that a person is driving under the influence of marijuana in CA, a prosecutor can use officer observations of driving patterns, observations during the traffic stop, performance on field sobriety tests, and the presence of THC in any blood test done.

Since “under the influence” is an extremely subjective standard, it is often very difficult to prosecute DUI of marijuana cases. This is especially true if the driver refused to perform the field sobriety tests and/or the officer did not observe driving that would be indicative of someone who is under the influence of marijuana.

If proposition 64 is passed, law makers could seek some sort of per se limit for how much THC can be in a person’s blood while driving. Several states have set a per se limit of five nanograms of THC per milliliter of blood. Colorado, has set a five nanogram per milliliter of blood limit to allow for the presumption that a person is “under the influence.” Unfortunately, current per se limits for THC, however, are an inaccurate measure of how impaired a person is. The same applies to a "breatholizer" test.

Perry Solomon, MD

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