Following up on my two previous posts that discussed chemical testing for driving under the influence of marijuana and marijuana per se laws, I think it would be interesting to the reader if I delved a bit more deeply into these topics. Driving under the influence of marijuana is sure to be a hot topic for the next few years as the legalization of recreational marijuana appears to be a trend.

I mentioned in my previous posts that unlike blood alcohol testing, testing for marijuana in a person’s system is much more problematic, at least as far as testing to determine if a driver was under the influence of the drug.

Urine testing will be positive for marijuana for 1 to 7 or more days after a person’s last use of the drug. A person can and likely will test positive for marijuana in a urine test even if he or she had not used the drug for days. Therefore, urine testing is unreliable proof that a person was driving under the influence. Besides that, while urine test can detect marijuana use, it does not detect THC, which is the psychoactive component in marijuana. There is no way to know by a urine test how much THC is in their system.

What about blood testing? Blood tests for the presence of marijuana in a person’s system is more reliable but there is no device that can be used in the field such as the breathalyzer which detects the amount of blood alcohol content in a person’s system. A blood test would have to be performed by an authorized technician, not the officer who makes the traffic stop. This would require, at the least, getting the person arrested for suspicion of driving under the influence of marijuana to the police station for a blood draw. Blood tests will show the presence of marijuana use for 12-24 hours after a person uses marijuana but if they are a regular user, the blood test will detect marijuana up to 7 days after the last use of the drug. Unlike urine tests, a blood test does measure the active presence of THC. As I noted in my post on marijuana per se laws, Colorado and Washington State both have driving under the influence of marijuana per se laws, which presumes that the person is under the influence of marijuana if he or she has 5 nanograms or more of THC per millimeter of blood.

But blood testing comes with its own set of problems. THC in the bloodstream rapidly declines in the blood stream after a person has inhaled. Within an hour, THC levels typically go from 100 ng/ml to single digit levels. It could take an hour or more between the traffic stop, field sobriety testing, arrests and transport to a location where a person’s blood may be drawn. By that time, the THC levels in the driver’s blood may be detectable but at very low levels.

On the other hand, low detectable levels of THC can show in a person’s blood for 8 hours or more.   The effects of the marijuana could have worn off many hours before a driver got behind the wheel, yet he or she could still register a low level of THC in his or her blood. The 5 ng/ml per se laws in Colorado and Washington State means that, just for example, someone could have smoked marijuana at noon and got behind the wheel at 5 p.m., long after the effects had worn off but would still be presumptively driving under the influence of marijuana if, as is perfectly possible, his or her blood THC content comes in at 5 ng/ml. For regular smokers of marijuana, the THC levels can linger longer and higher even days after the last use.

If someone has ingested marijuana (“edibles”), a method of marijuana use that is perhaps as frequently used as smoking it, blood testing for THC produces a different result. The level of THC in the blood rises over a couple of hours and then begins its decline.

Then there is the new technology involving saliva testing. Saliva testing will likely be the means by which the police will test for driving under the influence of marijuana testing in the future. As I noted in my previous post, hand-held oral swab testing devices are already being developed and tested. This technology has yet to be proven as reliable but the race is on to perfect these devices and indeed companies are already selling these devices to law enforcement.

At least one company is also marketing a hand-held marijuana breath test that police can use in the field and other companies are also developing such devices. These devices will detect THC in a person’s system but do not detect marijuana intoxication if a person has ingested the marijuana rather than smoked it. With the popularity of “edibles,” this is a big downside to the technology.

William Weinberg is an experienced DUI defense attorney. If you have been arrested for driving under the influence, whether under the influence of alcohol or drugs, you would be wise to consult right away with an attorney experienced in these matters. Please feel free to contact Mr. Weinberg to set up a confidential consultation without charge at or 949-447-8008.



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