Background To estimate the effect of social media use in 14-year-olds on risk of and inequalities in alcohol use and binge drinking at 17 years, using the UK-representative Millennium Cohort Study (born 2000–2002).

Methods The relationship between time spent on social media (using self-reports (n=8,987)) and time-use diaries (n=2,520) with frequency of alcohol use in the past month (never, 1–2, 3–5, and ≥6 times) and binge drinking was estimated using adjusted odds ratios (AOR) or relative risk ratios (ARRRs). Associations within low and high parental education groups were compared to examine effect modification. Analyses accounted for pre-specified confounders, baseline outcome measures (to address reverse causality), sample design, attrition and item-missingness (through multiple imputation).

Results Time spent on social media was associated with increased risk of binge drinking and alcohol use in a dose-response manner. Compared to 1<30 minute social media users, 30 minute-<1 hour users were more likely to report alcohol use ≥6 times/month (ARRR 1.62 [1.20 to 2.20]) and binge drinking (AOR 1.51 [1.22 to 1.87]), as were 1-<2 hour users (ARRR 2.61 [1.90 to 3.58]; AOR 2.06 [1.69 to 2.52]) and ≥2 hour users (ARRR 4.80 [3.65 to 6.32]; AOR 3.07 [2.54 to 3.70]). For binge drinking and alcohol use ≥6 times/month stronger effects were found for females (vs males). Social media measured by time-use diary was also associated with higher risks, although not always demonstrating a dose-response relationship. In social media users (vs non-users), there was a larger absolute risk of binge drinking in participants with high parental education (vs low). Analyses repeated in complete case samples, and with additional adjustment for baseline outcome measures revealed consistent findings.

Conclusion This is the first study to investigate the relationship between time spent on social media (using two measurement modes) on adolescent risk of alcohol use and binge drinking using representative longitudinal data, whilst accounting for confounders and potential reverse causality. Findings indicate increased time spent on social media is a cause of adolescent alcohol use and binge drinking. Our novel investigation of potential effect modification illustrates the impact of social media on health inequalities, where we found impacts on binge drinking were greater in adolescents from more socioeconomically advantaged households.

Lack of appropriate regulation of alcohol-related social media content may severely undermine public health messaging surrounding alcohol-related harms. In the absence of such regulation, findings indicate the requirement for guidance and tailored education supporting adolescents’ safe navigation of the social media environment.