In Vitro Comparison of Three Chairside Bleaching Protocols: Effects on Enamel Microhardness, Colour, and Qualitative Cytotoxicity Risk

<b>Background and Objectives:</b> The rapid increase of whitening products use raises questions about enamel safety. We compared three high-concentration protocols-Opalescence Quick (45% carbamide peroxide ≈ 15% H₂O₂), Opalescence Boost (40% H₂O₂), and BlancOne Ultra (35% H₂O₂ + LED)-under controlled conditions to balance color change (ΔE) with enamel integrity (microhardness, FTIR). We also constructed a qualitative cytotoxicity risk profile from published data; no biological assays were performed in this study. <b>Methods:</b> Seventy-two matched half-crowns were randomized to Control or one of the three protocols. Outcomes were a change in Vickers microhardness, spectrophotometric color difference, and FTIR carbonate-to-phosphate ratio after 24 h in artificial saliva. We also compiled a qualitative cytotoxicity risk profile from published evidence; no biological assays were performed. One-way ANOVA with Tukey HSD on Δ-scores, Shapiro-Wilk and Levene's tests for assumptions, Welch's <i>t</i>-tests for tooth-class comparisons, and Pearson correlation between ΔE and ΔMH. <b>Results:</b> All active protocols produced clearly visible whitening (mean ΔE 5.7-6.3). Hydrogen-peroxide gels showed greater hardness loss and carbonate depletion than the carbamide-peroxide gel under similar contact time. The association between greater shade change and hardness loss was moderate and not predictive for individuals. <b>Conclusions:</b> Under harmonized conditions, all systems whitened effectively. Pursuing changes beyond ~6 units offered little extra benefit while increasing enamel impact. Carbamide-based Opalescence Quick achieved comparable aesthetics with lower acute enamel effects. Clinicians should individualize exposure time and pair in-office whitening with short-term remineralising care. Cytotoxicity comments are qualitative and literature-based only.