Heavy metals present a paradox in biological systems—while elements like zinc, copper, and iron are essential micronutrients, others such as arsenic, mercury, cadmium, and lead are well known for their toxicity. Yet emerging evidence suggests that even these toxic metals can elicit adaptive or beneficial responses when present at ultra-trace levels, underscoring a complex, dose-dependent duality. This chapter synthesizes current understanding of this dichotomy, drawing on both recent literature and my own experimental research. In studies investigating aortic and tracheal hypercontractility, we found that arsenic, mercury, cadmium, and lead impair smooth muscle function via calcium dysregulation, oxidative stress, and altered adrenoceptor sensitivity. Conversely, other reports indicate that minimal exposure to these same metals can activate cytoprotective pathways—including antioxidant defenses and stress response signaling—suggesting a potential hormetic effect. Together, these findings emphasize the importance of nuanced toxicological assessment and open new avenues for exploring therapeutic thresholds in biomedical research.