Background Social media platforms, especially Facebook and WhatsApp, can spread public health information effectively. We aimed to estimate the influence of health-related messages circulated through these social media platforms on health-related decision-making and its associated factors. Methods We did a cross-sectional analytical study among adults (aged >18 years) who visited the outpatient department of a tertiary care hospital in suburban West Bengal, during July-September 2021. A structured questionnaire was used regarding receiving health-related messages on social media and the subsequent effect on health-related decision-making in the past year. Results A total of 673 individuals participated in the study. Their mean (SD) age was 34.4 (10.2) years and 56.8% (382) were men, 50.8% (342) were graduates, 63.6% (428) were from rural areas and 82.9% (558) were active users of more than one social media platform. A total of 474 (70.4%; 95% CI 67.0-73.9) study participants reported health-related decision-making based on social media messages, whereas 44.7% (301) reported checking the authenticity of forwarded messages or posts or updates with healthcare professionals before making a decision. On adjusted analysis, participants who had secondary education (adjusted prevalence ratio [aPR] 1.40; 95% CI 1.01-1.94), used both the media (aPR 1.31; 95% CI 1.09-1.58) and checked the authenticity of the messages with a healthcare professional (aPR 1.52, 95% CI 1.38-1.68) were significantly more influenced by the messages, posts or updates received on social media platforms. Conclusion WhatsApp forwards or updates and Facebook posts or updates influence health-related decision-making among the Indian adult population.