Description of studies    
    Of 17077 studies screened, 688 full text studies were    assessed, with 126 included (73 in the meta-analysis; fig 2). The    final sample included 1431534 adolescents (mean age of 15.0    years). Most included studies were cross-sectional (n=99; 79%)    and investigated high income countries (n=113; 90%),73 with 44    studies (35%) investigating US adolescents. Appendix 11 shows    the geographical distribution of included study populations.    Included and excluded study characteristics are presented in    appendix 11 and 12.  
            PRISMA flow diagram. APA=American Psychological            Association.*One study92 was not included in            the synthesis without meta-analysis (SWiM) as this            resulted in counting of study participants twice; we            were able to include estimates from this study in            meta-analyses stratified by outcome where this issue            did not occur          
    For 122 included cross-sectional and cohort studies, 57 (47%)    of studies were graded high risk of bias, 31 (25%) were    moderate, and 34 (28%) were low. Of the four randomised    controlled trials included, two were graded with some concerns    and two as low risk of bias (appendix 13). Reviewer risk of    bias agreement was strong (=0.91).79  
    Within included studies, many social media exposure measures    were reported, with most investigating multiple measures    (appendix 14). All were incorporated in our exploration of how    social media use is measured, therefore, the number of    datapoints reported differs across syntheses.  
    In total, 253 social media measures were reported: 135 (53%)    assessed frequency, 61 (24%) assessed exposure to content    displaying health risk behaviour, 45 (18%) assessed time spent,    and 12 (5%) other social media activities. Despite our broad    definition of social media, most included studies assessed a    narrow range of social media categories (or adopted a broad    definition). Social networking sites was the most common    category investigated (56%; n=141). Of those social media    measures investigating a specific platform (n=86), Facebook was    most investigated (n=40), followed by Twitter (n=10).  
    Of those 61 measures assessing exposure to content displaying a    health risk behaviour, 36 (59%) assessed marketer generated    content, 16 (26%) assessed user generated content, and nine    (15%) assessed both types of content. In total, 134 (53%) of    the 253 social media measures provided sufficient information    to differentiate between use that was active (eg, positing and    commenting on posts; n=90) or passive (eg, observing others,    content, or watching videos; n=44). Exposure ascertainment    primarily used unvalidated adolescent self-report surveys    (n=221) with a minority using data-driven codes, validated    adolescent self-report questionnaires and/or clinical records    (n=32).  
      Alcohol use was the most extensively studied outcome      (appendix 15). For time spent, 15/16 studies (93.8%) reported      harmful associations (95% confidence interval 71.7% to 98.9%;      n=100354; sign test P<0.001), 16/17 studies (94.1%) for      frequency (73.0% to 99.0%; n=390843; sign test P<0.001),      and 11/12 studies (91.7%) for exposure to content displaying      health risk behaviour (64.6% to 98.5%; n=24247; sign test      P=0.006). The category other social media activities was      investigated by one study (ie, participants had a Facebook      account) that reported a harmful association (95% confidence      interval 20.7% to 100%; n=4485; fig 3 for effect direction plot).    
              Effect direction plot for studies of the association              between social media use and adolescent alcohol use,              by social media exposure. Arrow size indicates sample              size; arrow colour indicates study risk of bias.              Sample size is represented by the size of the arrow,              measured on a log scale. Outcome measure is number of              outcome measures synthesised within each study.              Studies organised by risk of bias grade, study              design, and year of publication. Repeat              cross-sectional studies, multiple study populations              from different countries, and age subsets originating              from the same study reported as separate studies.              ESP=Spain; FIN=Finland; KOR=South Korea; NOS=assessed              via adapted Newcastle Ottawa Scale; RCS=repeat              cross-sectional study; SM=social media            
      In meta-analyses, frequent or daily (v infrequent or      non-daily) social media use was associated with increased      alcohol consumption (odds ratio 1.48 (95% confidence interval      1.35 to 1.62); I2=39.3%; n=383068; fig 4A). In      stratified analyses (appendix 16, p162-167), effect sizes      were larger for adolescents 16 years or older compared with      participants who were younger than 16 years (1.80 (1.46 to      2.22) v 1.34 (1.26 to 1.44); P<0.01 for test of      differences). Social networking sites were associated with      increased alcohol consumption, while microblogging or media      sharing sites had an unclear association (P=0.03).    
              Forest plots for association between frequency of              social media use and A) alcohol use, B) drug use, and              C) tobacco use. (A) Binary exposure (frequent or              daily v infrequent or non-daily) and binary              or continuous alcohol use outcome meta-analysis, with              OR used as common metric (N=383068). (B) Binary              exposure (frequent/daily v              infrequent/non-daily) and binary or continuous drug              use outcome meta-analysis, with OR used as common              metric (N=117645). (C) Binary exposure (frequent              v infrequent) and binary or continuous              tobacco use outcome meta-analysis, with OR used as              common metric (N=424326). Hard drugs were defined by              the cited papers as prescription drugs without a              doctors prescription (eg, OxyContin), cocaine crack,              methamphetamine, ecstasy, heroin, or opioids.              CI=confidence interval; ESP=Spain; FIN=Finland;              KOR=South Korea; OR=odds ratio; RoB=Risk of bias;              SM=social media; SNS=Social networking sites            
      Social media use for 2 h or more (v <2 h per day)      was associated with increased alcohol consumption (odds ratio      2.12 (95% confidence interval 1.53 to 2.95);      I2=82.0%; n=12390), as was exposure (v      no exposure) to content displaying health risk behaviours      (2.43 (1.25 to 4.71); I2=98.0%; n=14731; appendix      16, p168). Stratified analyses for time spent and exposure to      health risk behaviour content generally did not show      important differences by age and social media category      (appendix 16, p169-171). Associations were slightly stronger      for exposure to health risk behaviour content in user      generated (3.21 (2.37 to 4.33)) versus marketer generated      content (2.35 (1.30 to 4.22); P=0.28; appendix 16, p172).      Meta-analyses for frequency of use, time spent on social      media, and exposure to content displaying health risk      behaviour (assessed on a continuous scale) showed similar      findings (appendix 16, p173-174). On stratification (appendix      16, p175-179), for exposure to content displaying health risk      behaviour, associations were larger for adolescents 16 years      or older versus younger than 16 years (Std.Beta 0.35 (0.29 to      0.42) v 0.09 (0.05 to 0.13); P<0.001). The      results indicated that for every one standard deviation      increase in exposure to content displaying health risk      behaviour, alcohol consumption increased by 0.35 standard      deviation for older adolescents compared with 0.09 standard      deviation for younger adolescents.    
      For drug use, across all exposures investigated, 86.6% of      studies (n=13/15; 53.3% low/moderate risk of bias) reported      harmful associations (appendix 16, p180). The pooled odds      ratio for frequent or daily use (v infrequent or      non-daily) was 1.28 ((95% confidence interval 1.05 to 1.56),      I2=73.2%; n=117645) (fig 4B). Stratification showed no      clear differences (appendix 16, p182-184). Few studies (n=3)      assessed time spent on social media with estimates suggestive      of harm (odds ratio 1.45 (95% confidence interval 0.80 to      2.64); I2=87.4%; n=7357 for 1 h v >1      h/day) (appendix 16, p185).    
      For tobacco use, 88.9% (n=16/18; 50.0% low risk of bias)      studies reported harmful associations of social media use      (appendix 16, p 186). Frequent (v infrequent) use      was associated with increased tobacco use (odds ratio 1.85      (95% confidence interval 1.49 to 2.30); I2=95.7%;      n=424326) (fig 4C), as was exposure (v no exposure)      to content displaying health risk behaviours (specifically,      marketer generated content) (1.79 (1.63 to 1.96);      I2=0.00%; n=22882) (appendix 16, p188). In      stratified analyses (appendix 16, p189-193) for frequency of      use, stronger associations were observed for low and middle      income countries versus for high income countries (2.47 (1.56      to 3.91) v 1.72 (1.35 to 2.19); P=0.17), and for use      of social networking sites versus for general social media      (2.09 (1.72 to 2.53) v 1.48 (1.01 to 2.18; P=0.29).    
      Across all exposures investigated, 88.9% of studies (n=8/9;      77.8% low/moderate risk of bias) reported harmful      associations on electronic nicotine delivery system use      (appendix 16, p194). Exposure to content displaying health      risk behaviour (specifically marketer generated content)      (v no exposure) was associated with increased      electronic nicotine delivery system use (odds ratio 1.73 (95%      confidence interval 1.34 to 2.23); I2=63.4%;      n=721322) (appendix 16, p195). No clear differences were      identified on stratification (appendix 16, p196-197).    
      After excluding one study with inconsistent findings, across      all exposures investigated 90.3% (n=28/31; 67.7% high risk of      bias) reported harmful associations for sexual risk      behaviours (appendix 16, p 198). Frequent or at all use      (v infrequent or not at all) was associated with      increased sexual risk behaviours (eg, sending a so-called      sext, transactional sex, and inconsistent condom use) (odds      ratio 1.77 (95% confidence interval 1.48 to 2.12);      I2=78.1%; n=47280) (fig 5A). Meta-regression      (coefficient 0.37 (0.70 to 0.05); P=0.03) (appendix 16,      p276) and stratified analyses (appendix 16, p200-206)      suggested stronger associations for younger versus older      adolescents (<16 years v 16 years), but no      moderation effects were by social media category (P=0.13) or      study setting (P=0.49). Few studies assessed associations for      time spent on social media (appendix 16, p207).    
              Forest plots for association between frequency of              social media use and A) sexual risk behaviour, B)              gambling, C) anti-social behaviour, and D) multiple              risk behaviours. (A) Forest plot for binary exposure              (frequent/at all v infrequent/not at all)              and binary/continuous sexual risk behaviour outcome              meta-analysis, with OR used as common metric.              N=47280. (B) Forest plot for binary exposure              (frequent/at all v infrequent/not at all)              and binary/continuous gambling outcome meta-analysis,              with OR used as common metric. N=26537. (C) Forest              plot for binary exposure (frequent/at all v              infrequent/not at all) and binary/continuous              anti-social behaviour outcome meta-analysis, with OR              used as common metric. N=54993. (D) Forest plot for              binary exposure (frequent/at all v              infrequent/not at all) and binary/continuous multiple              risk behaviours outcome meta-analysis, with OR used              as common metric. N=43571. CI=confidence interval;              n=Number of study participants; OR=odds ratio;              RoB=Risk of bias; SM=Social media; SNS=Social              networking sites            
      After excluding one study that had inconsistent findings,      across all exposures investigated, all six studies      investigating gambling reported harmful associations      (appendix 16, p208). Frequent or at all use (v      infrequent or not at all) was associated with increased      gambling (not via social media) (odds ratio 2.84 (95%      confidence interval 2.04 to 3.97); I2=85.6%;      n=26537) (fig 5B). On differentiation by social media      category, a relatively large association was found for online      gambling via social media (3.22 (2.32 to 4.49)), however,      associations were not present for social networking sites and      general social media (appendix 16, p211).    
      Across all exposures investigated, all 16 studies (43.8%      low/moderate risk of bias) that investigated anti-social      behaviour showed harmful associations (appendix 16, p212).      Frequent or at all use (v infrequent or not at all)      was associated with increased anti-social behaviour (eg,      bullying, physical assault, and aggressive/delinquent      behaviour) (odds ratio 1.73 (1.44 to 2.06);      I2=93.3%; n=54993) (fig 5C), with time spent similarly      associated with increased risk (appendix 16, p214). No      subgroup differences were noted (appendix 16, p215-217).    
      For inadequate physical activity, after excluding three      studies with inconsistent findings, 36.4% of studies (n=4/11;      72.7% low/moderate risk of bias) reported harmful      associations across all exposures investigated (appendix 16,      p218). No association between time spent on social media      (assessed on a continuous scale) and adolescent engagement in      physical activity was seen (Std.Beta 0.00 (95% confidence      interval 0.02 to 0.01); I2=59.8%; n=37417)      (appendix 16, p219), with no important differences across      subgroups (appendix 16, p220-222).    
      Across all exposures investigated, all 13 studies (including      four randomised controlled trials: two rated low risk of bias      and two some concerns) that investigated unhealthy dietary      behaviour showed harmful associations, with most at low risk      of bias (61.5%) (appendix 16, p223). Exposure to health risk      behaviour content (specifically marketer generated content)      was associated with increased consumption of unhealthy food      (odds ratio 2.48 (95% confidence interval 2.08 to 2.97);      I2=0.00%; n=7892) when compared with adolescents      who had no exposure (appendix 16, p224-225).    
      For multiple risk behaviours, all nine studies showed harmful      associations across all exposures investigated (appendix 16,      p226). The pooled odds ratio for frequent and at all social      media use (v infrequent and not at all) was 1.75      ((95% confidence interval 1.30 to 2.35); I2=97.9%;      n=43571) (fig 5D), but the few studies precluded      stratification.    
      For electronic nicotine delivery system use, associations      were stronger for cohort study datapoints (odds ratio 2.13      (95% confidence interval 1.72 to 2.64) v 1.43 (1.20      to 1.69) for cross-sectional datapoints; P=0.004) (appendix      16, p228) but no clear differences were seen for other      outcomes (appendix 16, p229-240). Although based on few      studies, for unhealthy dietary behaviour a stronger      association was found for the randomised controlled trial      datapoint versus for the cross-sectional datapoints (3.21      (1.63 to 6.30) v 2.48 (2.08 to 2.97); P=0.44)      (appendix 16, p241).    
      When stratifying by adjustment for critical confounding      domains, no clear differences were identified (appendix 16,      p242-253), with some exceptions. Associations were stronger      for unadjusted versus adjusted datapoints for exposure to      content displaying health risk behaviour and alcohol use      (Std.Beta 0.28 (0.14 to 0.43) v 0.07 (0.03 to 0.12);      P=0.008) and for frequent (v infrequent) social      media use and alcohol use (odds ratio 1.54 (95% confidence      interval 1.36 to 1.78) v 1.34 (1.24 to 1.44);      P=0.06) (appendix 16, p254-255).    
      For alcohol use, effect sizes were generally stronger for      moderate and high risk of bias datapoints (v low)      (appendix 16, p256-257), excluding time spent (2 v      <2 h per day) and exposure to health risk behaviour      content (v no exposure) where low (compared with      moderate and high) risk of bias datapoints displayed stronger      associations (appendix 16, p258-259). For drug use and sexual      risk and anti-social behaviour, no differences were      detectable or low/moderate risk of bias datapoints showed      stronger associations (compared with high) (appendix 16,      p260-264). For tobacco use and gambling, stronger      associations were found for high risk of bias datapoints or      no clear differences were identified (appendix 16, p265-267).      No clear differences by risk of bias were observed for the      remaining outcomes (appendix 16, p268-269).    
      When we excluded datapoints that overlapped the age range of      10-19 years, a marginal reduction in effect size (appendix      16, p270) or no important differences were noted (appendix      16, p271-274).    
      Funnel plots and Eggers test results suggested some      publication bias in the meta-analysis investigating frequent      or at all social media use (v infrequent or not at      all) and sexual risk behaviours (P=0.04; bias towards the      null) (appendix 17). Insufficient data precluded      investigation of other outcomes.    
      As frequency was the most investigated exposure, and      continuous and binary exposures reported similar effects, we      focused the GRADE assessment on the binary exposure of      frequency of use. We report harmful effects on alcohol use      with low certainty, and with drug, tobacco, electronic      nicotine delivery system use, sexual risk behaviours,      gambling, and multiple risk behaviours with very low      certainty.    
      We conducted a post-hoc GRADE assessment for exposure to      content displaying health risk behaviour (v no      exposure) and unhealthy dietary behaviour because of the      substantial difference in quality of evidence observed (four      randomised controlled trials); we report moderate GRADE      certainty (table 1, appendix 18).59    
            Condensed summary of findings and certainty of evidence            (as per GRADE)          
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Social media use and health risk behaviours in young people: systematic review and meta-analysis - The BMJ