A challenge with universal school-based mental health supports is the limited understanding of potential unintended or unanticipated outcomes. In this review, we examined 47 academic and gray literature sources to address the question, "What are the side effects of universal school-based mental health supports?" We discuss how universal supports can positively impact student mental health, enhance school staff's knowledge and attitudes in addressing mental health topics, and contribute to an improved school climate. However, universal supports can also lead to school staff feeling the strain of resource and time pressures from integrating mental health programming into demanding schedules, voicing frustrations about or exhibiting resistance to mental health supports, and encountering varied, unpredictable outcomes for different student populations across system contexts.