BACKGROUND: A social media campaign for mothers aimed at reducing indoor tanning (IT) by adolescent daughters reduced mothers' permissiveness toward IT in an immediate posttest. Whether the effects persisted at 6 months after the campaign remains to be determined. METHODS: Mothers (N = 869) of daughters ages 14-17 in 34 states without bans on IT by minors were enrolled in a randomized trial. All mothers received an adolescent health campaign over 12 months with posts on preventing IT (intervention) or prescription drug misuse (control). Mothers completed a follow-up at 18 months post-randomization measuring IT permissiveness, attitudes, intentions, communication, and behavior, and support for state bans. Daughters (n = 469; 54.0%) just completed baseline and follow-up surveys. RESULTS: Structural equation modeling showed that intervention-group mothers were less permissive of IT by daughters [unstandardized coefficient, -0.17; 95% confidence interval (CI), -0.31 to -0.03], had greater self-efficacy to refuse daughter's IT requests (0.17; 95% CI, 0.06-0.29) and lower IT intentions themselves (-0.18; 95% CI, -0.35 to -0.01), and were more supportive of bans on IT by minors (0.23; 95% CI, 0.02-0.43) than control-group mothers. Intervention-group daughters expressed less positive IT attitudes than controls (-0.16; 95% CI, 0.31 to -0.01). CONCLUSIONS: The social media campaign may have had a persisting effect of convincing mothers to withhold permission for daughters to indoor tan for 6 months after its conclusion. Reduced IT intentions and increased support for bans on IT by minors also persisted among mothers. IMPACT: Social media may increase support among mothers to place more restrictions on IT by minors.