In this open-label semi-naturalistic study, simulated and perceived driving performance among 40 patients was assessed prior to and following self-administration of their own prescribed medical cannabis product. While oil users tended to have higher SDLP values, this was stable over time and there was no evidence of impairment for either administration route. Furthermore, the lack of changes in speed variability suggests a modest but sustained stabilisation of vehicle control. Interestingly, there was also a significant increase in average speed at 5 h, in comparison to both baseline and the 2.5 h mark, with this increase in speed reflecting a greater accuracy in adhering to the assigned speed limit. Critically, no notable evidence of driving impairment (i.e. a significant decline in driving performance metrics within the simulated driving scenario) was observed for either consumption modality, relative to baseline.

Contrary to our findings, Di Ciano et al. (2020) observed reductions in mean speed during simulated driving scenarios requiring higher cognitive load but not during highway driving conditions. Preceding studies primarily involving cannabis naïve users have also more frequently associated acute cannabis use with an increase in speed variability and a general decline in average speed when using smoked or vaporised forms of cannabis (Brands et al., 2019; Lenné et al., 2010; Ronen et al., 2008). Decreases in average speed are often inversely and dose dependently related to THC concentration. A comprehensive review by Neavyn et al. (2014) suggests that this behaviour is largely attributed to drivers’ attempts to compensate for attention lapses when they are conscious of the subjective effects linked with cannabis-induced intoxication. In our study, a marginal increase in average speed was observed, this increase did not exceed the instructed speed limit of 100 km/h. This increment of <1 km/h could also be more simply attributed to participants’ desire to complete the task swiftly after a long day, despite the scenario being timed rather than distance based. This contrasts with the habitual over-cautious driving behaviour of cannabis-intoxicated individuals, who typically exhibit compensatory or unnecessary reductions in speed. Notably, such caution is more characteristic of occasional cannabis users who are acutely intoxicated. By contrast, patients in our study exhibited only mild intoxication, potentially reflecting increased tolerance or familiarity with cannabis’ effects (Ramaekers et al., 2020). This may explain the relatively stable speed and reduced likelihood of speed variability exhibited, which aligns with findings by Doroudgar et al. (2018) and suggests an adaptation to driving demands and a departure from the heightened caution seen in less frequent users.

The absence of a discernible increase in SDLP, a measure frequently considered sensitive in assessing driving performance (Hartman and Huestis, 2013), was somewhat unexpected. This is especially so, given substantial prior research demonstrating that THC consumption reliably increases SDLP in both simulator and on-road paradigms (Arkell et al., 2020b; Bosker et al., 2012; Hartman et al., 2015; Ramaekers et al., 2000). Even at relatively low THC dosages of 5 mg, a statistically significant, although not ecologically substantial, increase in vehicle weaving remains discernible compared to placebo (Manning et al., 2023). These dosages are markedly below those used in the current study (9.61 mg for oil products vs 37.00 mg for flower products), and also significantly lower than those commonly employed in a myriad of other experimental driving studies (Arkell et al., 2020b).

To contextualise our findings within an ecological framework, it is noted that an increase in SDLP of 2.4 cm is generally considered ‘ecologically significant’, as this degree of deviation is comparable to that observed at a blood alcohol concentration of 0.05% (Verster and Roth, 2011). Noteworthily, absolute SDLP values in this patient group across all drives (spanning 28.95–29.13 cm) display little difference compared to what has been recorded in healthy volunteers during a placebo condition (27.41 cm) under identical driving scenario conditions and similar demographic characteristics (Manning et al., 2023). Differences in SDLP between healthy placebo-controlled volunteers and the medical cannabis patients in our study, along with the fluctuations in SDLP among these patients over time, did not meet this established ‘ecologically significant’ threshold.

Interestingly, patients who self-administered oil products exhibited a higher SDLP overall relative to those who used flower products. The persistence of differences in SDLP at baseline suggests that the variations observed between treatment modalities may not be directly attributable to product exposure. Initially, those using oil-based products reported exerting significantly more effort while driving relative to those using flower-based products. This difference gradually declined, with oil users reporting a considerable decrease in effort by their final drive, aligning them more closely with flower product users. This change indicates that for those consuming oil products, the perceived mental or physical demand for driving decreased over time. This reduction in perceived effort during later drives could be attributed to adaptation to the oil-based product’s effects, or simply greater familiarity with the driving task; although it is also possible that flower users may have already become accustomed to the effects of their prescribed vaporised product prior to the post-dosing driving tasks. It is important to consider that factors such as individual differences in drug metabolism and tolerance may influence self-reported perceived effort levels and that these self-reported effort levels do not necessarily correlate with actual driving performance.

The prolonged acute intoxication associated with oral THC products and the more gradual onset of effects may partly explain the differences in drug-effect profiles (Vandrey et al., 2017). Users who consumed their treatment via vaporisation may have experienced diminished drug effects by the time they commenced the driving task. Variations in daily or nightly usage patterns could also have impacted these results, as patients who ingest their medical cannabis orally the night before (i.e. as a sleep aid) may have experienced residual effects the following morning. The observed disparities in baseline performance between different routes of administration may also be partially attributed to the high variability in both oral fluid and whole blood THC concentrations, particularly among oil users.

A previous report indicated that driving impairment following THC vaporisation may linger for up to 4.5 h in healthy users (Marcotte et al., 2022). By contrast, the utilisation of medical cannabis for intractable health conditions may attenuate the adverse impacts on cognitive and psychomotor functions often associated with the clinical symptoms of these conditions (Makhoul and Jankovic, 2023). This is particularly evident in chronic cannabis users who exhibit fewer neurocognitive deficits compared to infrequent users (Ramaekers et al., 2009). Existing research supports these findings, albeit derived primarily from self-reported evidence, which suggests a therapeutic role for medical cannabis in managing clinical symptoms commonly associated with cognitive and psychomotor impairment (Eadie et al., 2021; MacCallum et al., 2022). Specifically, patients who routinely administer medical cannabis for chronic conditions may develop a tolerance to the prototypical THC-induced impairments in driving abilities that are more pronounced in occasional or inexperienced users, thereby potentially reducing the impact of THC on driving competency seen in otherwise healthy subjects (Celius and Vila, 2018; Freidel et al., 2015; Veldhuijzen et al., 2006).

Prior research suggests that recreational and/or infrequent THC use can impair driving performance; however, this effect is thought to be less pronounced in habitual users, particularly patients who adhere to a stable and regular dose of a prescribed medical cannabis product (MacCallum and Russo, 2018). It is posited that tolerance to the effects of THC among chronic users may result in significantly reduced intoxication and/or impairment with a given dose of THC relative to occasional cannabis users (Doroudgar et al. 2018; Ramaekers et al., 2021). A similar mitigation of impairment could therefore extend to medical cannabis patients who are undergoing long-term treatment and adhere to maintaining a steady dosing regimen. As with other psychoactive medications, consistent and stable dosing may minimise instances of impaired driving, with tolerance development further mitigating potential impacts on driving competence (Verster and Mets, 2009). The effective management of chronic health conditions may offset potential negative symptomatic effects, leading to limited changes or even improvements in driving performance (Celius and Vila, 2018).

Outside of treatment effects related to dose, mode and timing of acute cannabis administration, the development of cannabis tolerance may attenuate certain acute driving and psychomotor performance deficits (Sevigny, 2021). More recent evidence from higher dose and more frequent cannabis users suggests the role of pharmacodynamic models of tolerance, where repeated cannabis exposure leads to neuroadaptive responses dulling acute effects (Colizzi and Bhattacharyya, 2018; Ramaekers et al., 2020). The absence of clear driving impairment in the current study, despite performance variations between routes of administration, likely indicates at least some mitigation of drug effects due to symptom relief and a potential acquired tolerance to THC’s impairing effects. It remains important to consider that the observed effects may be, in part, influenced by patients being highly motivated to demonstrate their driving is not impaired by cannabis. While this suggests that patients can overcome THC-related impairment that might have otherwise been present in an observational study setting, this still may not fully represent the real-world driving scenario, where such motivation to demonstrate unimpaired driving is less pronounced. Lastly, an absence of observable driving impairment could be due to the relatively low THC doses in medical cannabis products consumed or that patients’ driving abilities were compromised prior to consumption of their medical cannabis product.