The primary objective was to evaluate the comparative effects of loneliness on multiple distinct health outcomes. The literature was qualitatively reviewed to identify loneliness risk factors, explore mechanisms, and discuss potential evidence-based interventions for targeting loneliness. 114 identified studies were systematically reviewed and analyzed to examine for associations between loneliness (as measured by the UCLA Loneliness or de Jong Gierveld Loneliness Scales) and one or more health outcome(s). Health outcomes were broadly defined to include measures of mental health (i.e., depression, anxiety, suicidality, general mental health), general health (i.e., overall self-rated health), well-being (i.e., quality of life, life satisfaction), physical health (i.e., functional disability), sleep, and cognition. Loneliness had medium to large effects on all health outcomes, with the largest effects on mental health and overall well-being; however, this result may have been confounded by the breadth of studies exploring the association between loneliness and mental health, as opposed to other health outcomes. A significant effect of gender on the association between loneliness and cognition (i.e., more pronounced in studies with a greater proportion of males) was also observed. The adequate training of health care providers to perceive and respond to loneliness among patients should be prioritized.