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Marijuana: Does it Cause Cognitive Impairment During Driving?

Evidence has shown that cannabis is associated with an increased risk of motor vehicle accidents, especially when combined with alcohol. Presented here is one clinician’s guide for screening for cannabis in a chronic pain practice.
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Twenty-three states and the District of Columbia currently have legalized medical marijuana and 10 states completely have decriminalized the substance. Our goal as healthcare providers is to improve our patients’ health and well-being, to replace maladaptive behaviors with more adaptive coping skills, and to educate patients about behaviors or activities that put them or others at risk for adverse outcomes.

I believe that the use of marijuana in chronic pain patients receiving opioid analgesics, or other controlled substances, puts them, and others, at significant increased risk for adverse outcomes, especially in settings that require complete attention, alertness, and mental acuity. These skills are required while driving and frequently necessary at the workplace. In addition, prescribing medications to patients who use marijuana may put you, the prescriber, and your medical practice at increased risk for lawsuits.

Flickr: FotoSleuthFollowFord Falcon crash

This feature will briefly review the evidence regarding the efficacy of marijuana, as well as safety concerns, with a focus on how marijuana can adversely affect a patient’s ability to operate an automobile (see My Policy on Marijuana).

Marijuana’s Efficacy

Clinical research has demonstrated that marijuana has efficacy in the management of intractable nausea, anorexia, and vomiting in cancer patients undergoing chemotherapy, HIV/AIDS patients, and cachexic patients, as well as in other debilitating medical disorders.1-3 There also are multiple studies documenting marijuana’s efficacy with some intractable pain disorders.

However, there is a difference between a therapeutic dose of medicinal marijuana in the form of Marinol and tetrahydrocannabinol (THC) from smoking marijuana—both in terms of known dosage, potency, and purity.2 The amount of THC absorbed in Marinol is known; in contrast, the dose of THC absorbed via smoking marijuana varies—and by all accounts, is much stronger than in the past.2

Reviews of the pharmacology of marijuana indicate that the percentage of THC contained in current marijuana cigarettes often is many times greater than its counterpart from the 1960s and 1970s.4 This finding is important because the effects of THC are dose-related. Ashton noted that most of the research on cannabis was performed in the 1970s using doses of 5 to 25 mg THC.4 In prior studies, subjects may have been given marijuana cigarettes with between 1.5% and 4% THC, whereas marijuana on the street today may contain between 10% and 30%, or more, of THC. Because of the higher concentration of THC in today’s marijuana, impairment levels are higher than those noted in past studies.

My bias has come from personal experience with patients, as well as scientific studies. A large body of data demonstrates that marijuana may cause significant mental status changes, including altered perception with occasional hallucinations and delusions, euphoria and dysphoria. We must consider the issue of risk to the individual smoking marijuana not only from immediate usage but also long-term effects, including the possibility of substance abuse.

Safety of Today’s Marijuana: Gateway Drug?

My clinical practice consists of a high-risk population, including current or former substance abusers. When I reviewed my patients’ early drug abuse history, which included childhood/adolescent experimentation, it strongly suggested that for the majority of patients in this population, marijuana was a gateway drug that either led to more regular marijuana abuse or, more commonly, to the use of more potent substances including opiates, heroin, and cocaine.

This opinion was confirmed by clinical evaluation as well as responses on follow-up questionnaires of these patients. Patients were also asked to respond to the following questions:

  • Did your early-life (preadolescent or adolescent) use of alcohol or marijuana serve as a gateway drug, making it more likely that you would use a more potent drug later in life?
  • Do you think early-life (preado-lescent or adolescent) use of alcohol or marijuana results in an increased use of more potent drugs later in life?

In both instances, the overwhelming responses (>75%) indicated their belief that alcohol or marijuana were gateway drugs.5

My own experience was confirmed in an observational study by Pesce et al.6 In that study, 21,746 urine specimens were obtained from chronic pain patients. The investigators noted that up to 13% of the patients tested positive for the acid form of THC. Moreover, there was a roughly 4-fold incidence (odds ratio, >3.7) in the use of cocaine and methamphetamine among marijuana users in this patient population.6

Taking this into consideration, I revisited this issue with many of my high-risk pain patients and the majority agreed that legalizing recreational marijuana for individuals over age 18 can be expected to result in major psychosocial problems as well as significant added risks in the workplace, in many activities requiring full attention and concentration (eg, riding bicycles and other athletic activities), as well as an increase in driving and work accidents. Their comments also reinforce my conviction that many children, adolescents, and young adults are likely to be exposed to recreational marijuana more readily than would have been the case when there were established legal sanctions against marijuana use.

Editor’s Note: In Colorado and Washington State, the legal age for purchasing marijuana is 21, similar to the legal drinking age.

Risks vs Benefits: THC and Driving

It is now clear that marijuana plays a significant role in motor vehicle accidents (MVA) across the United States. Studies of acute THC use have found a 50% increase in risk of MVAs.7 As many as 33% of drivers tested at the scene of accidents were positive for marijuana and another 12% tested positive for marijuana and cocaine, according to studies reviewed by Brookoff et al and Sanderstom et al.8,9

Hartman and Huestis studied the effects of cannabis on driving skills and the evidence pointed to a significant increased risk in MVA when participants presented with 2 to 5 ng/mL of THC in their systems.10 The results of this study also revealed a significant need to consider decreased reaction time and increased distractibility in both regular THC users and occasional users.

Ramaekers et al also reported an increased crash risk among THC users, likely due to impairment of cognitive and psychomotor skills, as well as actual driving performance.11 More recent studies revealed that THC exposure significantly decreased psychomotor function on simulated driving tests and critical task taking (CTT) instruments. CTT is used to detect any impairment present regardless of the causation (ie, fatigue, alcohol, or cannabis intake).

Last updated on: October 7, 2015
First published on: September 1, 2015
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My Policy on Marijuana