The results of our study corroborate research showing the detrimental effect of alcohol use on injury. The results for cannabis use were quite surprising, as they were associated with less risk of injury, which seems to contradict the laboratory studies cited above, as well as opposes the observations on alcohol use. While these findings are thus more in line with other epidemiological studies showing no effect of cannabis use, possibly even a protective one (e.g., [32, 35]), they are limited by the small sample size of cannabis users and do not necessarily contradict findings from laboratory studies. One possibility is that persons driving under the influence of cannabis become more cautious in real-life driving situations than they would in the laboratory [10, 24], e.g., by avoiding potentially risky situations [10]. Furthermore, the present study did not only look at traffic casualties, but also included other mechanisms for injury with potential links to cannabis use that are different from those for traffic injuries.

The compensation hypothesis is unlikely to be the only explanation of our results. First, as is the case for alcohol, there should be limits for such compensation [27, 28]. The present study in fact indicated a 'protective effect' of cannabis use in a dose-response relationship. Second, the combined intake of alcohol and cannabis failed to show an increased risk for injury when compared with unaccompanied alcohol use, as should be expected [21]. Nonetheless, the relative risk was below even 1 when compared with individuals that abstained from both substances in the six-hour period prior to injury. Third, compared with other studies (e.g., [16, 21]) only very few (i.e. 2.9% of the present sample) had consumed cannabis in the six-hour period prior to injury. This seems to be quite low given the fact that Switzerland has one of the highest cannabis use prevalence rates in the world (see [36] for students, and [37] for adults), and therefore may point to the possibility that Swiss cannabis users take precautions to avoid injuries while using cannabis.

Another possible explanation is that when compared to alcohol consumption, cannabis is consumed in relatively safer, low risk environments (e.g. at home, private locations, not public such as bars or while 'going out'), independent of whether also it is consumed with alcohol. Future studies should look at the environment of predominant cannabis usage and whether the place of consumption (e.g. at home, during special occasions) is differentially associated with risks of injuries. The number of cannabis usage cases in the present study was simply too small, to permit a stratified analysis by place of consumption.

As the main results of the study are based on self-reported measures of cannabis use, there may have been a deliberate denial of illicit behaviour. Although this may explain the small prevalence, it is unlikely to explain a protective association with injury. It also begs the question of why individuals would deny cannabis use before an injury, but admit use in the week prior (which is needed to result in a relative risk below 1)? As shown above, there is not a lot of indication for deliberate denial. However, there may be other memory effects, e.g., the ability for recall may have been impeded by alcohol and cannabis use, especially for the period of one week before the injury. Recall errors may have also particularly affected the estimates in case-crossover studies (e.g., overestimation of relative risks due to the underestimation of use in the control period that is retrospectively farther away in time [38]), but would not explain why cannabis use had a protective effect.

While the participation rate was relatively high (8% refusals), we cannot exclude the possibility, that exclusion may have been associated with exposure. Another limitation of our research was the small sample size for cannabis use, and as a result, we should be cautious with conclusions. Clearly, more research is needed, epidemiological studies in particular. Roadside surveys in the tradition of Borkenstein and colleagues [3] should be conducted more systematically to allow for a better examination of the causal effects of cannabis use in traffic injury. In such studies, not only is the blood alcohol concentration in participants of traffic injuries measured, but a random sample of comparable traffic participants is measured as well. This allows for the establishment of relative risk estimates. However, even if such studies yield significant relative risks for injury as a consequence of cannabis use, this does not necessarily demonstrate sufficient evidence for the public health importance of the problem. It may be that although cannabis is causally connected to traffic injury, the importance of this risk factor is small due to a low prevalence of people combining cannabis with other risk factors, such as drinking and driving. Thus, attributable fractions may be small. Of course, the public health importance may differ in regions with higher prevalence of cannabis use. One of the more interesting research continuations in this area would be to replicate this study in regions with higher prevalence of cannabis use and higher incidence of injury (e.g., some parts of Africa or the Caribbean).

In conclusion, we need a more systematic exploration of the relationships between cannabis use and other forms of injury than traffic injuries. There are some indications of cannabis use having a negative biological link to aggression [39], hence many categories of injuries may be differentially impacted by cannabis as compared to alcohol. These relationships should be studied in real life settings and not only in the laboratory, as other factors such as deviance or stigmatization may play a role.

Public health researchers tend to disregard subjective reports of cannabis users, seemingly indicating that they are more cautious and avoid risky situations subsequent to use (e.g., [40]). However, given the numerous reports on different mechanisms of injury, we should take these reports more seriously and should start exploring more systematically which situations involving cannabis use are associated with increased or decreased risks of injury. Such research could not only shed light on important interactions between social and biological determinants of behaviour, but could also contribute to better preventing cannabis-related harm.