Cannabis Use in Older Individuals May Be an Important and Under-Recognized Risk Factor for Motor Vehicle Crashes

Introduction

More older people are trying and consuming cannabis regularly.^1–3 Such use is both recreational (now that such use is legal or decriminalized in many states) and medical (in this population particularly to address insomnia, painful conditions, and aid anxiety.) ⁴ Rates of recent cannabis use are increasing more dramatically in older adults compared to younger ones.^1,2 Some 7% of those over 65 and 10% of those ages 60–64 reported past-month cannabis use in 2023, up from less than 1% in 2006 and 5% in 2022, and compared to 24% of young adults.^2,5 Relevant background factors include increased cultural acceptance of cannabis and commercial interests promoting its therapeutic benefits. Older individuals who do use cannabis are more likely to consume it in edible form. ⁶ As well as cannabis’ purported health benefits, adverse consequences associated with use of the substance, including those related to automobile driving, may differ in older individuals.

The most recent available figures show that in 2022, there were 57.8 million Americans aged 65 and older, representing 17.3% of the total U.S. population, a percentage that continues to grow. ⁷ There are 51 million licensed drivers in the United States in this age group, approximately 22% of all licensed drivers nationally. This number has substantially increased over the years, with a 77% increase since 2004. ⁸ The proportion of fatal crashes involving older drivers follows a similar trend, rising from 11% in 2001 to 19% in 2021. ⁹

Elements of aging can adversely impact driving, ¹⁰ such as age-related changes in vision (comprising diminished response to glare, visual acuity, and depth perception) affecting the ability to judge distances and detect hazards. Decreased speed of movement and range of motion due to a variety of commonly occurring conditions can interfere with steering, braking, or turning the head to check blind spots. Reduced reaction time can make it harder to respond to sudden changes in traffic or road conditions. In addition, age-related alterations in the ability to reason and remember, as well as medications (more of which are taken by older persons), likely affect some older adults’ driving abilities.^11,12 Various medications commonly used by older adults can have side effects that impair driving ability.

Although older adults typically drive fewer miles and less frequently, their crash rates per mile driven tend to be higher than all but the youngest drivers and are more likely to be involved in intersection crashes, especially when turning across traffic. Older drivers experience more severe injuries and fatalities per thousand vehicle crashes even at lower speeds, in part because they tend to be frailer, with less physical resilience and thus more vulnerable to injuries. According to one review, the risk for fatal crashes is 3–20 times higher for older compared to non-older drivers, per mile driven.^9,13

Taken together, increasing rates of cannabis use in older individuals, more older people, plus the larger numbers of older, more vulnerable vehicle drivers raise important safety questions about whether this situation represents a substantial risk to public health.

To summarize briefly cannabis’ effects on driving, the drug’s adverse behavioral consequences are mainly attributable to its THC content, with cannabidiol (CBD) neither playing a substantial independent role in driving impairment nor preventing THC-induced impairment of driving and cognition.^14–16 Broadly, at least in young and middle-aged individuals, cannabis affects driving adversely following acute consumption, acutely (and potentially chronically), in a dose-related manner.^17,18 More specifically, acute cannabis exposure affects a wide variety of cognitive and perceptual processes that are critical for driving, including reaction time, memory, processing speed, executive function, attention, and divided attention.^19–26 In addition, some evidence suggests that cannabis may particularly adversely affect cognitive functioning in older individuals following both acute and chronic use.^27–32

Focusing on driving, the existing literature documents many driving-while-high effects on closed roadway^33–36 and driving simulator assessments.^37–46 These data clearly show that cannabis alters driving. But these studies have by-and-large been focused only on the use of a rather small number of outcome measures (e.g., over-reliance on standard deviation of lane position, a measure of weaving). Thus, while much evidence suggests that acute cannabis use adversely affects both simulated and on-road driving ability,^15,47–49 until recently many more complex, key driving domains remained unexplored. ⁴⁹ In addition, (and crucially), age-related effects remain unstudied.

There are important qualifiers to the above summary. These include effects of tolerance in regular users and inter-individual susceptibility to the drug.^50,51 The literature on chronic cannabis use and driving is mixed and conflicting.^52,53 Finally, not all studies agree that THC is more cognitively disabling in older populations: some research suggests cognitive benefits from THC in older rodents.^54,55

Nevertheless, results from the above experimental driving studies accord with real-world observations. Epidemiologically, cannabis use is associated with substantially increased risk of motor vehicle crashes, with odds ratios of at least 1.4, and THC is more likely to be detected in the blood of seriously or fatally injured road users (the latter does not necessarily indicate acute use).^56,57 Some evidence suggests that cannabis involvement in motor vehicle crash fatalities rises in tandem with increasing use of cannabis in those states that legalize the drug.^58–60 For example, in Canada, following national legalization of recreational cannabis, a relatively sharp overall increase in traffic-injury emergency department (ED) visits occurred during the commercialization phase (from March 2020 onward) when retail availability expanded. This rise far exceeded changes in alcohol-involved ED visits during the same time period. ⁶¹ Post-legalization, in British Columbia trauma centers, the largest increases in moderately injured drivers evaluated who tested above legal THC levels were observed among drivers 50 years and older, with an adjusted prevalence ratio of 5.18 after legalization. These statistics documented that older drivers experienced a much sharper relative rise in this measure compared to younger age groups.^29,30 However, not all studies are consistent with these observations. Also, less specifically, a variety of insurance industry and National Highway Traffic Safety Administration (NHTSA) data suggest that any drug use in older drivers is associated with increased crash involvement.^62–64

Integrating the above information, while there are many reasons, though, to expect that driving abilities may be particularly compromised following acute cannabis use in older individuals, there has been a failure to document empirically how severely cannabis worsens that risk.

Underlying reasons for more marked driving compromise in an older population include the possibility of THC’s interaction with decreased cognitive function in this age group. For example, because memory, learning, attention, processing speed, inhibition, decision-making, and cognitive flexibility, domains impacted by acute cannabis exposure, also all decline with age ⁶⁵ older adults may be differentially vulnerable to these acute effects, as supported by some recent studies. ⁶⁶

Acute cannabis use also adversely impacts age-related visual processes,^20,67 (affecting ability to judge distances and detect hazards) and on gait stability^68,69 (related to motor vehicle collision involvement in older drivers). ⁷⁰ In addition, age influences interactions with both alcohol and with medications often taken by older people (e.g., benzodiazepines for insomnia), which likely potentiate intoxicating effects, resulting in greater impairment of alertness, balance, coordination, and reaction time, further increasing the likelihood of falls and crashes. ⁷¹ In animal models, aging affects endocannabinoid/alcohol interactions. ⁷² However, all these plausible interactions between aging and acute cannabis use require support by actual acute challenge studies in older individuals.

In terms of potential mechanisms, age-related relevant physiological influences tending to make older adults more sensitive to psychoactive effects of cannabis include altered body composition and enzyme activity leading to possible pharmacokinetic alterations, that is, THC persistence. These are especially germane for cannabis ingested via the oral route, an important consideration given the preference for this form of cannabis consumption in the older population. ⁷³ Polypharmacy and the potential for prescribed medications to interact with cannabis are additional risk factor. For example, cumulative anticholinergic burden is associated with cognitive decrements. ⁷⁴ Finally, there are reduced numbers of cannabinoid receptors with increasing age.^75,76 In summary, existing evidence suggests that older adults may be more vulnerable to cannabis-related driving impairment, and consequently, their crash risk is likely elevated.

Despite the above concerns, as noted, there are remarkably few studies that quantify age-related cannabis effects on driving ability, either from the epidemiology literature or with regard to on-road or simulated driving studies, highlighting the need for research on aging, cannabis-using drivers.^64,67,77

In addition to these normal aging processes affecting older individuals, a significant percentage of persons in this age group suffer from pathological brain aging conditions, including mild cognitive impairment (MCI) and early dementia syndromes. A 2023 study estimated that nearly 7.5 million older Americans over age 65 (approximately 13%) are likely living with MCI, many of whom are unaware of their diagnosis. ⁷⁸ Roughly, one to two out of 10 such individuals are estimated to develop dementia over a 1-year period. ⁷⁸ A majority of those with MCI continue to drive, as do a substantial proportion with different stages of dementia. ⁷⁹ Cognitive processing speed in individuals with MCI and mild dementia is strongly related to driving skills.^79,80 Although it is a reasonable assumption that drivers with some degree of neurocognitive compromise would be particularly vulnerable to the effects of cannabis on crash risk, few if any research studies have sought to quantify this.

Although an overarching model remains to be articulated, a reasonable hypothesis is that aging amplifies THC’s effects on driving-related abilities, for example, by greater impairment of postural stability, cognition, vision, sleep, and alertness. This amplification of acute THC-related impairment effects may be mediated through a series of underlying, interconnected, well-documented, age-related alterations. These include metabolic inefficiencies that may result pharmacokinetically in higher and more prolonged THC levels,^62,81–83 reductions in CB1 receptor availability and broader endocannabinoid changes may heighten sensitivity to any given THC dose,^84–86 and brain networks more vulnerable to THC’s complex, system-wide disruptions. ⁸⁷ Common health-related comorbidities in older adults may further compromise brain function or functional capacity, thereby increasing safety risks. ⁸⁸ Collectively, these factors suggest older adults may respond more strongly and adversely to THC than younger cohorts.^89,90 A possible counterargument is that cannabis use for medicinal reasons in older individuals (e.g., for pain or sleep) might improve symptoms and functioning, with possible secondary benefits to driving ability. It is precisely the lack of such direct published comparisons of older versus younger individuals responding to an acute cannabis challenge, that bespeaks the urgent need for targeted research.

Recommendations

Cannabis use in older drivers is a potentially substantial and growing public health problem. This potentially problematic topic with notable public health implications is certainly understudied and needs to be better addressed through appropriately designed research studies. Beedham et al. (2020), noted that, “… quality of the research is weak, and few older patients have been enrolled in cannabinoid studies…high quality research is urgently needed…”. ⁹¹

What we do know currently supports a research agenda to fill those gaps and quantify older adults’ risk for problem driving while using cannabis, then determine how that risk is modified/made worse by the multiple age-related changes we summarized above. Fundamental issues needing clarification include quantifying the differential impairing effects of typically consumed doses of cannabis/THC in different age groups on both on-road and simulated driving, using rigorous experimental designs (e.g., placebo-controlled, randomized double-blind challenge studies). Other fundamental issues to be explored are age-related differences in pharmacokinetics of orally administered THC preparations (the preferred route of cannabis consumption for older individuals) and differential impairment due to cannabis/alcohol combinations.^42–46 Co-consumption of the two substances is fairly common, with an estimated 15–30% of adults now often co-administering both concurrently.^92,93 Differential impairing effects of cannabis/THC on driving in individuals with and without cognitive impairment (MCI, dementia) also require adequate documentation. Given the propensity of older individuals to use orally administered CBD, pharmacokinetic interactions with oral THC and resulting effects on cognition and driving should be documented in this population. However, given the lack of effects of THC/CBD combinations of inhaled cannabis on driving, this risk is theoretical until proven otherwise.^14–16

Knowledge about current risks as well as the findings from these proposed studies should, we believe, be transmitted to older individuals through their geriatric primary care providers. These latter individuals need to educate adults about cannabis (i.e., enhance the standard model of PCP care for older adults). Older adults who used cannabis in their youth need to be reminded that the cannabis of today is much more potent than that from the past with which they are more likely to be familiar. Public and scientific awareness needs national public health messaging from health providers extending all the way down to local dispensaries. In addition to warning labels on products (paralleling those reminding of specific risks to children), product labels should draw attention to driving risks in older individuals. Finally, it is important to bear in mind that continued ability to drive in older adults is associated with independence and quality of life, so that testing and regulation of older drivers is always a balance between public safety and autonomy.