Many people now watch health videos, such as diet, exercise, mental health, COVID-19, and chronic disease videos, on social media. Most existing studies focused on video creators, leaving the motivations and practices of viewers underexplored. We interviewed 18 participants, surveyed 121 respondents, and derived a model characterizing consumers' video consumption practices on social media. The practices include five main activities: deciding to watch videos driven by various motivations, accessing videos on social media through a socio-technical ecosystem, watching videos to meet informational, emotional, and entertainment needs, evaluating the credibility and interestingness of videos, and using videos to achieve health goals. Through an iterative video consumption process, individuals strategically navigate across multiple platforms, seeking better accessibility, higher reliability, and cultivating a stronger motivation. They actively look for longer and more in-depth videos. We further identified challenges consumers face while consuming health videos on social media and discussed design implications and directions for future research.