How do laws against driving under
the influence of marijuana work?
Blood testing seems to be the only
reliable method to determine the actual level of THC in the body, since urine tests
cannot show that a person has recently used marijuana. Depending on quantity
and strength, a single dose of THC produces metabolites in urine that last for
at least 12 days — long after the psychoactive effects of the substance have
worn off.1 However, the key is not necessarily to know the exact
level of THC in a driver’s bloodstream, but whether or not the driver is
impaired and thus incapable of safely operating a motor vehicle.
What is the threshold for
considering a driver to be impaired by marijuana?
It is unclear what blood level of
THC (the main psychoactive ingredient in marijuana) constitutes actual
impairment. Most credible scientists working on the issue acknowledge the
difficulty of pegging THC impairment to a number (in a way similar to drunk
driving laws), and epidemiological evidence on the risk of accidents associated
with marijuana is much less conclusive than data regarding alcohol.
The most meaningful recent study
measuring driver "culpability" (i.e., who is at fault) in 3,400
crashes over a 10-year period indicated that drivers with THC concentrations of
less than five ng/mL in their blood have a crash risk no higher than that of
drug-free users.2 The crash risk begins to rise above the risk for
sober drivers when a marijuana user's THC concentrations in whole blood3
reach five to 10 ng/mL.
How long does it take for the
psychoactive effects of marijuana to wear off?
Because smoked THC is rapidly
transferred into the blood stream, THC levels in the blood rise quickly
immediately after inhalation. Depending on the dose, THC typically reaches peak
concentrations of more than 100 ng/mL five to 10 minutes after inhalation and
then rapidly decreases to between one and four ng/mL within three to four
hours.
However, heavy marijuana users’ blood
can contain detectable amounts of THC even after periods of abstention. In one
controlled study, six of 25 participants tested positive for active levels of
THC after a full seven days of abstention, with the highest concentration
detected being three ng/ml of whole blood.4 In addition, the blood
serum of heavy to moderate users may contain more than two ng/mL of THC at 24
or even 48 hours after smoking a single joint, a level that studies have shown
does not produce impairment.5
This is a particular concern for
medical marijuana patients who are using marijuana in compliance with state
laws and their doctors' advice, but who would likely test positive for
marijuana while sober. While the Colorado Legislature debated a per se THC limit of five ng/ml, Denver News’ medical marijuana reviewer
(and medical marijuana patient), William Breathes, subjected himself to blood
draws to test his THC levels. After a 15-hour period of abstinence, Mr.
Breathes’ THC levels were still 13.5 ng/ml. According to his physician, Mr.
Breathes was in “no way incapacitated” at the time.6 This first-person
account demonstrates the very real possibility that medical marijuana patients
and other heavy marijuana users could face criminal charges under a per se system even if they are not
actually impaired.
The graphic below shows the mean
plasma levels of THC and its metabolites (11-OH-THC and THCCOOH) for six
subjects smoking a marijuana cigarette containing 34 mg of THC, following
several days of abstinence (which would reflect an occasional user's pattern of
usage).7
Additionally, several studies show
that exposure to secondhand marijuana smoke (which could result from being in
the same room with a person who is using marijuana) may cause a non-user to
show THC concentrations in blood serum of several nanograms per milliliter.8
Does this mean that some DUID laws
may actually criminalize sober drivers?
Yes, if they set a per se limit for THC. Furthermore,
arresting and convicting motorists who only have marijuana metabolites in their
systems (from having used marijuana days or weeks before) will certainly cause
people who are completely sober to be arrested and wrongly convicted of driving
under the influence of drugs.
The standard for scientists is to
test blood and urine, but what about other bodily fluids, like saliva, or
performance-based tests?
Because of the invasiveness of blood
tests and the inadequacy of urine tests in determining impairment on the
roadside (i.e., actual THC levels), police officials hope to institute roadside
saliva testing in the near future. However, the technology for reliably testing
saliva is still unavailable, and there are no national standards for testing
saliva, as there are with blood and urine.
Significant work is being done to
develop and implement modified field sobriety tests, which measure the behavior
of drivers (reaction time, for example), rather than their bodily fluids.
MPP recommends a policy similar to
most state laws on driving under the influence of alcohol: A driver who fails a
roadside sobriety test should be required to submit to a blood test by a
trained medical professional — or risk criminal and administrative sanctions.
However, given the length of time after impairment THC can stay in a person’s system,
no conviction for driving under the influence of marijuana should be based on a
per se limit.
1 B. Law, et al., "Forensic
aspects of the metabolism and excretion of cannabinoids following oral
ingestion of cannabis resin," Journal of Pharmacy and Pharmacology,
1984, 36(5), pp. 289-94.
2 O.H. Drummer, et al., "The
involvement of drugs in drivers of motor vehicles killed in Australian road
traffic crashes," Accident Analysis and Prevention, 2004, 36, pp.
239-48.
3 "When reporting THC levels in
blood or adopting legal limits one must always specify the reference fluid …
For THC, concentrations measured in whole blood are typically 1.6-2.2 times
lower than those measured in blood or plasma. For example, 5 ng/mL of THC in
whole blood correspond to about 10 ng/mL in serum or plasma." (F.
Grotenhermen, et al., "Testing for Impairment by Cannabis," 2004.)
4 Erin L. Karschner et al., “Do Δ9-Tetrahydrocannabinol
Concentrations Indicate Recent Use in Chronic Cannabis Users?,” Addiction, 2009, 104(12), pp. 2041-2048.
5 G. Skopp, et al., "Serum
cannabinoid levels 24 to 48 hours after cannabis smoking," Archiv fur
Kriminologie [German publication], 2003, 212(3-4), pp. 83-95.
6 William Breathes, “THC blood test:
Pot critic William Breathes nearly 3 times over proposed limit when sober,” Denver News, April 18, 2011.
7 Huestis, et al., “Absorption of THC
and formation of 11-OH-THC and THC-COOH during and after smoking marijuana,” Journal of Analytical Toxicology, 1992,
16, pp. 276-82.
8 E.J. Cone, et al., “Contact highs
and urinary cannabinoid excretion after passive exposure to marijuana smoke,” Clinical Pharmacology and Therapeutics,
1986, 40, pp. 247-256.
