An Mlive article recently explained the findings of a Michigan special commission tasked with making a recommendations about THC limits and driving.
Presently, Michigan does not have a “legal limit” for THC and driving, which essentially means someone can be prosecuted for having any amount of THC in their blood while driving, but the burden would be on the prosecutor to prove that the person was impaired, intoxicated, or high. This is obviously different from drinking and driving, where a blood/breath alcohol level of 0.08 and up is per seenough to support a conviction (note, however that a person can still be convicted for a BAC of less than 0.08). This is also different from the higher schedule drugs, where any amount of the drug in someone’s system is enough to support a conviction.
Which means marijuana use makes for a very tricky subject with respect to OUI/DWI and presents a very real danger of legal consequences for someone that uses marijuana products and then drives before the THC is completely out of their system. In a journal article available through the National Center for Biotechnology Information, the authors reported that:
Maximum ▵9-THC plasma concentration was observed approximately 8 minutes after onset of smoking, while 11-OH-THC peaked at 15 minutes. . . . This ▵9-THC concentration rapidly decreases to 1-4 ng/mL within 3-4 hour[s].
In comparison to smoking and inhalation, after oral ingestion, systemic absorption is relatively slow resulting in maximum ▵9-THC plasma concentration within 1-2 hours which could be delayed by few hours in certain cases.
So, according to that article, an average person that inhaled an average dosage of THC would have their THC peak in 8 minutes and then decrease to 1-4ng/ml 3 to 4 hours later. Thus, in states such as Washington and Colorado, which have a per se THC limit of 0.05 ng/ml, average users might be safe from prosecution if they wait 4 hours before driving. But in states such as Michigan, the risk of prosecution would exist until the amount of active THC decreases to zero, which takes hours or days longer.
I have previously noted the distinction between active and inactive THC. I have also argued at length with prosecutors about that difference. Inactive THC, or carboxy THC, is generally what is referred to when people talk about THC being detectable by drug tests for 30 days or even months. Inactive THC can take a long time to be completely eliminated. Michigan does not allow conviction based solely on inactive THC. Active THC, or the type of THC that actually has a psychoactive effect on the user, is eliminated much more quickly, as discussed above. However, the rate of elimination trails off after the initial rapid decrease, and thus a user might have a low amount of active THC in their system much longer. Active THC might be present in low blood concentrations in chronic users for days or more. Thus, without a per se limit, THC users seem to be at risk of prosecution for impaired/intoxicated driving for a indeterminate amount of time that may be much longer than they are aware of.
The Mlive article also explains “[a]fter reviewing several studies, the commission found there is no scientifically supported threshold for THC that correlates to impaired driving.”
Blood levels of THC can be low when people are very impaired — and conversely, chronic users of marijuana with high levels of THC in their blood can not be impaired, according to the commission’s report. With multiple ways of consuming marijuana — from smoking it to eating infused edibles — the time it takes to feel its effects vary widely and from person to person.
And the Mlive article listed the six states that presently have a per se limit:
Six states have established per se limits for THC impairment. Colorado, Montana and Washington have set theirs at 5 nanograms per milliliter (ng/mL). Nevada and Ohio have limits of 2 ng/ML, and Pennsylvania has a legal limit of 1 ng/ML.
The commission’s report is available here and certainly provides useful reference materials for persons interested in marijuana law and DUI/OWI.
As a final reminder: Michigan legalized using and/or possessing marijuana in certain cases, but it is still illegal to drive while impaired by marijuana.