# Cytotoxicityicity of Bleaching Products: A Systematic Review
## 1 Introduction  
In recent years, there has been a trend in dentistry towards aesthetic treatments like dental bleaching, aimed at lightening tooth color to enhance smiles. This procedure involves applying chemical agents that oxidize the organic pigmentation of the tooth, resulting in a lighter shade [1, 2]. The mechanism of action involves the breakdown of oxidizing agents into free radicals, which then attack pigmented molecules, reducing their light reflection and producing a whitening effect [3]. The efficacy of this technique varies depending on the extent of tooth discoloration [4, 5]. Tooth color, a critical aspect of smile attractiveness, is defined by three dimensions: hue, chroma (saturation), and value. [PERSON] and colleagues have proposed adding a fourth dimension, opacity/translucency. Tooth color is a critical aspect that determines the attractiveness of an individual's smile [6]. Traditionally, we can speak of intrinsic staining, which occurs after a change in the composition or thickness of the dental tissue [7]. On the other hand, extrinsic discoloration can be direct and exhibit a multifactorial etiology. It is produced by chromogens derived from food ingested in the diet or placed habitually in the mouth, such as tobacco, coffee, or tea. Or it can also be indirect, in which the compounds do not have the color of the resulting stain, such as antisetics or metallic salts [8]. Understanding these physiological and pathological processes involved in tooth staining allows for individualized treatment approaches. There are several bleaching agents on the market, such as peroxides. Hydrogen peroxide (HP) is the active agent and the main component which is responsible for the chemical action. It is a strong oxidizing agent that produces free radicals, reactive molecules, and HP ions [4]. In contrast, carbamide peroxide (CP) is a stable complex formed by urea, which stabilizes the pH, and releases HP by decomposition when it encounters water. The former has a longer active permanence and is used in the office when staining is moderate or severe. The latter is used in home treatment, in cases of milder discolorations [9]. Treatment methods include in-office procedures, home treatments prescribed by dentists, or a combination of both. Home treatment involves using custom trays with CP gel for a set duration, following an initial in-office application. In-office procedures often employ higher bleaching agent concentrations, such as 35-40% HP, to achieve faster or more intense results [10; 11]. To minimize sensitivity, various materials like remineralizers or desensitizers, e.g., sodium fluoride or casein phosphopeptide amorphous calcium phosphate (CPP-ACP), are used, along with gingival protection barriers like antioxidants (vitamin E or N-acetylcysteine) [12; 13; 14; 15]. Sensitivity and gingival irritation are common adverse effects of bleaching, influenced by the agent's penetration ability, concentration, and application time [16; 17]. In addition to the sensitivity and/or gingival irritation, bleaching can produce relevant iatrogenicity at the cellular level, known as cytotoxicity [18; 19]. The oxidizing agent can affect the pulp, producing pathological stress which may lead to necrosis or direct damage to cell membranes. On the other hand, it can cause damage to fibroblasts, generating oxidative deterioration in them [20]. This deterioration can lead to cell death and mutations [21]. This cytotoxicity is evaluated through cell viability studies in fibroblasts and dental pulp stem cells [22]. According to ISO 10993-5, if a material has cell viability values below 70%, it is considered cytotoxic [23]. The most used assay is the MTT assay (3-[4; 5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide). Its main objective is to calculate cells that are viable in a high yield. A reduction in the number of mitotic cells provides evidence of cell growth inhibition. Some of its advantages are its quick measurement and analysis and the incorporation of several samples [24].