Several recent reviews have confirmed an association between psychosis and cannabis use (1–6), while a few have failed to identify a statistically significant relationship (7, 8). The strength of associations identified in meta-analyses ranged from strong for the most severe cannabis users [(5): OR 3.90] to moderate [(8): OR 1.786; (3): RR 1.71; (4): OR 1.75 for abuse/dependence; (5): OR 1.97 for any use] or weak [(7): RR 1.11; (4): OR 1.14 for lifetime use]. Several recent reviews have also confirmed an association between psychosis and tobacco use (8–11), with the strength of the association ranging from moderate to strong in meta-analyses (respectively, OR 2.22, 3.04, 3.22/2.18 for case-control/prospective studies, and RR 1.99). One review did not find evidence of any significant association (12). Those reviews that have compared the strength of the relationship between cannabis and psychosis to that between tobacco and psychosis have found that the latter appears to be somewhat stronger (8, 13). The associations between psychosis and poor diet quality or sedentary behavior have not been extensively studied, although reviews have suggested that people with psychosis tend to have poor diet quality (14, 15) and low levels of physical activity (16, 17).

A number of recent review articles support an association between cannabis use and depression, although with some mixed results (12, 18–28), while one found that the relationship is unclear (29). The meta-analysis by Onaemo et al. (26) identified a strong association between cannabis use disorder and major depression (OR 3.22), while those by Esmaeelzadeh et al. (18) and Gobbi et al. (19) as well as an earlier meta-analysis by Lev-Ran et al. (30) identified a weak to moderate association (respectively, OR 1.29, 1.37 and 1.62) for overall cannabis use.

For tobacco, recent reviews by Weinberger et al. (31) and Fluharty et al. (32) confirmed that people with depression are more likely to smoke and to meet criteria for nicotine dependence, with the latter also indicating that smoking may lead to depression. A meta-analysis by Esmaeelzadeh et al. (18) found that the association between tobacco use and depression was moderately strong with an OR of 1.65, which was stronger than that for the corresponding association between cannabis use and depression. Another meta-analysis by Groenman et al. (33) found that childhood depression was a risk factor for subsequent nicotine use disorder (OR 2.56). Reviews by Zeng and Li (34) and Han et al. (35) also confirmed that secondhand smoking is associated with depression, with their meta-analyses obtaining ORs of 1.60 and 1.32, respectively. Furthermore, recent reviews have also indicated that e-cigarette use is associated with depression (36, 37).

There are also a number of reviews supporting associations between depression and sedentary behavior (17, 38–47) and poor diet quality (48–57), although with some mixed results. In meta-analyses, the associations were weak to moderate in strength, with RR 1.42 (47) and RR 1.25 (46) for overall sedentary behavior, OR 1.28 (45) and OR 1.12 (42) for screen time, RR 1.18 for TV viewing (41), OR 1.44 for ultra-processed food consumption (49), RR 1.25 (58) and RR 1.31 (48) for sugar-sweetened beverage consumption, OR 1.18 for western-style unhealthy dietary pattern (59), and OR 1.62 (52) and OR 1.11 (54) for fast/junk food.

Some reviews have identified an association between cannabis use and anxiety (18, 20, 26, 27, 60–62), although others have maintained that the relationship is unclear (19, 22, 28, 29, 63, 64). In meta-analyses, moderate to strong associations were identified between cannabis use disorder and anxiety [(60): OR 1.68; (26): OR 2.99], while the associations for overall use were generally weak [(18): OR 1.36; (19): OR 1.18; (60): OR 1.24; (61): OR 1.15; (62): OR 1.25]. Reviews have also identified an association between tobacco use and anxiety (12, 18, 65–67), although with some mixed results. A meta-analysis by Esmaeelzadeh et al. (18) found a strong association between tobacco use and anxiety with an OR of 2.21, which was stronger than that for the corresponding association between cannabis use and anxiety.

Reviews have also found evidence of an association between anxiety and sedentary behavior (68–71) and poor diet quality (49, 52). Associations in meta-analyses were of weak to moderate strength, with regular physical activity reducing the odds for anxiety [(72): OR 0.54; (70): OR 0.74] and sedentary behavior [(68): OR 1.48], ultra-processed food [(49): OR 1.48] and junk food [(52): OR 1.24] increasing the odds for anxiety.

The relationship between cannabis use and bipolar disorder has not been intensively studied, but a number of reviews have suggested a positive association (20, 23, 25, 73–78). Meta-analyses by Gibbs et al. [(73): OR 2.97] and Hunt et al. [(74): OR 2.35] identified a strong association between cannabis use disorder and bipolar disorder. Some reviews have also suggested an association between tobacco use and bipolar disorder (75, 79), although the evidence is mixed (12). Nevertheless, the meta-analysis by Jackson et al. (79) identified a strong association with an OR of 3.5. Reviews have also identified tentative associations between bipolar disorder and sedentary behavior (17, 80, 81) and diet quality (82, 83), but there are few available studies.

While the association between personality disorders and general substance use is well established, the evidence specifically for cannabis (20, 28, 84–86) and tobacco (12, 87) is quite limited. Similarly, the research into the relationships between personality disorders and sedentary behavior and diet quality is very limited (87, 88).

ADHD is commonly diagnosed in childhood, and most studies investigating the relationship between the disorder and substance use focus on ADHD as a risk factor for subsequent substance use. For cannabis, meta-analyses have found strong associations between childhood ADHD and regular [(89): OR 2.45] and lifetime use [(90): OR 2.78] as well as a moderate association for abuse or dependence [(90): OR 1.58]. For tobacco, meta-analyses similarly identified childhood ADHD as a strong risk factor both for subsequent nicotine use [(89) OR 2.16; (90): OR 2.08] and nicotine dependence or use disorder [(33): OR 2.52; (90): OR 2.82]. The evidence for e-cigarette use is very limited (36).

Reviews have also supported associations between ADHD and sedentary behavior (91–95) and diet quality (96–99). Meta-analyses of sedentary behavior found moderate to strong associations between electronic media use and ADHD [(91): OR 1.94; (92): 2.597], while meta-analyses of diet quality found weak to strong associations for sugar and soft drink consumption [(97): pooled effect size 1.22], junk food [(99): OR 1.51], and general unhealthy diets [(96): OR 1.41; (98): OR 2.24].

Reviews have identified an association between cannabis use and PTSD (100, 101) and investigated the potential for therapeutic use (102–104). Reviews have also identified an association between tobacco use and PTSD (105–109), with one meta-analysis by van den Berk-Clark et al. (109) finding a strong association (OR 2.13) for unadjusted studies and a weak association (OR 1.22) for a single adjusted study. The evidence for e-cigarette use is very limited (36).

Furthermore, reviews have suggested that both sedentary behavior and poor diet quality are associated with PTSD, although with some mixed results (70, 109–111). In meta-analyses, high physical activity was moderately protective against PTSD [(70): OR 0.57], while PTSD in turn was a weak risk factor for low physical activity [(109): OR 0.91 for adjusted studies and OR 0.69 for unadjusted studies]. The meta-analysis by van den Berk-Clark et al. (109) on diet quality was somewhat unclear, as four unadjusted studies predicted a healthier diet (OR 1.25) while a single adjusted study predicted a less healthy diet (OR 0.95).

While these reviews indicate a broad range of associations between unhealthy behaviors and mental disorders, they also demonstrate that such associations have not often been understood in relation to each other across various behaviors and disorders, which would allow for both a calibration of their relative importance and a widened perspective on their etiology. Instead, individual associations are usually analyzed and discussed in isolation, which encourages the perspective that the association under scrutiny is unique and reflects a particular and individual etiology. Should it be true that the range of disorders here under scrutiny are generally associated with a range of unhealthy behaviors related to diet, exercise, and licit and illicit substance use, however, it might seem unlikely that these parallel associations are best understood in isolation from one another. If the association between a given behavior and a given mental disorder has parallels across a range of disorders and behaviors, its lack of specificity indicates that the association probably does not reflect a causal effect from the behavior itself (112). Instead, such a field of associations most likely point to general and shared dynamics. One such dynamic may involve underlying issues of social misery and psychological trauma that perhaps give rise to both unhealthy behaviors and to mental disorders. To take one example, long-term unemployment may entail poverty, boredom, and social isolation, which in turn may push a person into unhealthy behaviors as means of escapism and diversion and also contribute to a mounting sense of hopelessness and depression.

Indeed, there is strong evidence supporting a relationship between mental disorder and factors relating to misery and trauma. To take a few noteworthy examples, childhood abuse has been associated with psychosis at ORs of 7.3 (113) and 5.9 (8), with depression at ORs of 19.1 (114) and 4.2 (115), and with ADHD at ORs of 7.3 (91) and 3.3 (116), as well as with cannabis abuse at ORs of 5.8 (117), 2.7 (118), and 2.2 (119). Violence victimization has similarly been associated with psychosis at ORs of 5.9 (120), 4.0 (121), 2.2 (122), and 1.9 (123), with PTSD and depression at ORs of 32.4 and 5.4 (124), and with nicotine dependence at ORs of 22.4 (124) and 5.5 (125). Afifi et al. (118) for their part found that living in an unsafe community was associated with cannabis use at an OR of 5.0 and with tobacco use at an OR of 3.8. Comparable findings exist for childhood bullying victimization (120, 126), while poverty, low education, and unemployment have been associated with psychosis at ORs of 8.2 (127) and 3.0 (122), with depression at ORs of 9.1 (128), 5.0 (129), 3.4 (130), 3.2 (131), 2.8 (132), and 2.6 (133), with nicotine dependence at ORs of 37.0 (134), 2.4 (135), and 2.3 (125), and with cannabis use disorder at an OR of 2.5 (136). It should also be noted that factors relating to trauma and misery might accumulate in an individual and thereby increase risk. In a study by McMahon et al. (120), cumulative adverse life events were strongly associated with psychotic experiences, with four or more adverse life events incurring an OR of 16.8.

Thus, it seems possible that both unhealthy behaviors and mental disorders might be caused by underlying problems related to social misery and psychological trauma, and that the observed associations between those behaviors and disorders are largely spurious. As a basis for the discussion of the specificity of individual associations, this article will use the probably most discussed associations among those here under purview, namely that between cannabis use and psychosis, which is commonly taken to reflect a causal effect from cannabis use. A recent consensus paper by D'Souza et al. (1) pointed to a study by Starzer et al. (137) as evidence supporting the specificity of this association, and on this basis drew the inference that “[c]annabis is more likely to be associated with psychosis outcomes than other psychiatric diagnoses” (p. 732). This consensus paper also pointed to a somewhat broader range of evidence to support the specificity of the cannabis-psychosis association as compared to other forms of substance use, drawing the inference that “[t]he risk for a psychosis outcome is highest for cannabis relative to other drugs” (ibid.).

However, there is also evidence contradicting the consensus view presented in D'Souza et al. (1). As Ksir and Hart (138) pointed out, research has associated cannabis use with depression, which may challenge the specificity of the cannabis-psychosis association. Furthermore, a recent review by Johnstad (13) found that tobacco use tended to be at least as strongly associated with psychosis as is cannabis use. The cannabis-psychosis association may therefore lack specificity both across disorders and across behaviors, which would open for the perspective that the association may be spurious and reflective of a general etiology perhaps related to underlying psychological trauma and social misery.

This interpretation is supported by a study by Shakoor et al. (139) which found that in a sample of adolescent twins, “[e]nvironmental influences explained all of the covariation between cannabis use and paranoia, cognitive disorganization and parent-rated negative symptoms,” with a bivariate common environment of 69%−100% (p. 144). Cannabis use explained 2%−5% of variance in positive, cognitive, and negative psychotic experiences. Sideli et al. (140) for their part found that less than daily cannabis use was not associated with psychosis (OR 0.97, p = 0.92) unless it co-occurred with having experienced child abuse, in which case the association was strong (OR 2.51, p < 0.05), while Copeland et al. (141) found that cannabis use before the age of 16 was strongly associated with depression (OR 2.3) and anxiety (OR 2.9) in an analysis adjusted only for sex and race/ethnicity, but that the association disappeared with adjustment for childhood disorders and adversities (OR 0.6 for both depression and anxiety). A meta-analysis by Fusar-Poli et al. (8) similarly found that risk for psychosis was related to environmental risk factors such as childhood trauma and further accumulated by low education and unemployment, while finding no significant impact from cannabis use.

In sum, the worry that unhealthy behaviors such as cannabis use cause psychosis may be a case of projecting the social characteristics of the drug-using population onto the drug itself and understand these characteristics as pharmacological effects inherent to the drug use. This comparative review aimed to assemble data on a broad range of associations between unhealthy behaviors and mental disorders in order to compare their respective strengths and to gain insight into the influence of factors relating to trauma and misery by recording the inclusion of relevant control variables and subsequently running sensitivity tests on the dataset. It reviews recent (2015 through January 2023) research on associations between a range of unhealthy behaviors and mental disorders, using the much-debated association between cannabis use and psychosis as a basis for comparison. In order to obtain comparable figures, the review included only records that presented their findings as odds ratios, and attempted to account for variations in behavior intensity, sample generalizability, and covariates via subsequent sensitivity analyses. By thus gaining a comparative overview of the relationship between unhealthy behaviors and mental disorders, the review hoped to achieve three specific aims:

The review focused on moderate cannabis and tobacco use as compared to high levels of sedentary behavior and poor diet quality. This approach was founded on the observation in Johnstad (13) that heavy substance use is very likely associated with underlying factors of social misery and psychological trauma, and that analyses of the heavy drug-using population are therefore at high risk of confusing the social characteristics of this population segment with the effects from the drug use itself. While heavy chronic drug use is commonly driven by coping motives related to underlying misery and trauma, moderate use is driven by a much broader range of motivations (143–145). In order to understand pharmacological effects from substance use in and of themselves, it therefore seems advisable to focus on moderate usage patterns.

In this review, moderate cannabis use was defined as one or two use occasions per week (146, 147). For tobacco, moderate use could be defined as daily use of about 10 cigarettes (148–150). When the studies included in this review reported effects related to several different usage patterns, the one most closely resembling a moderate pattern was selected. The review did not exclude any study on the basis of divergences in the usage pattern, however, and several included studies focused exclusively on use disorders or similarly problematic behavior. For sedentary behavior and diet quality, the review focused on high exposure represented by such measures as the worst quartile, daily consumption of soda or fast-food, not meeting physical activity guidelines, or more than 4–8 h of screen time per day. This approach was based on the observation that unlike cannabis and tobacco use, sedentary behavior and unhealthy food consumption are universal behaviors with which moderate levels of engagement are ubiquitous. When studies reported figures from several different multivariate models, the review included the figures from the final, most stringently adjusted model. Sensitivity analyses attempted to account for the inclusion of studies focusing on specialty samples and disordered behavior patterns.

From 3,682 records, 294 were included. Some publications reported figures for several behaviors and methodological approaches (commonly, both cross-sectional and longitudinal), and counting these the review included 97 studies of cannabis use, 132 studies of tobacco use, 88 studies of sedentary behavior, and 47 studies of diet quality. The dataset included a total of 570 associations between behaviors (cannabis use, tobacco use, sedentary behavior, and diet quality) and disorders (psychosis, depression, anxiety, bipolar disorder, personality disorder, ADHD, and PTSD).