# Low-Level Laser Therapy for Tooth Sensitivity after Tooth Bleaching: A Systematic Review
## 1 Introduction  
Tooth bleaching (TB) is a popular cosmetic procedure known for its effective whitening results, minimal patient harm, and non-invasive nature. It can be performed either in-office by a dentist or at home using custom trays, with both methods yielding significant results when used properly. In-office TB is often preferred for its ability to deliver rapid, substantial, and longer-lasting whitening, effectively addressing severe tooth discoloration under professional supervision. Discolored teeth are usually treated with high concentrations of hydrogen peroxide (HP, H\({}_{2}\)O\({}_{2}\)), typically ranging from 35% to 38%, due to its potent oxidative properties [1]. The outcome depends on the concentration of the bleaching agent, the duration of application, the temperature, and the pH of the bleaching solution [2, 3]. While hydrogen peroxide is the most common bleaching agent, carbamide peroxide is also used; it breaks down into hydrogen peroxide and urea, with hydrogen peroxide performing the bleaching action. Hydrogen peroxide decomposes to form reactive oxygen species (ROS), such as hydroxyl radicals OH\({}^{-}\), singlet oxygen O\({}_{2}\)\({}^{-}\), and superoxide anion O\({}_{2}\)\({}^{-}\), which interact with chromophore molecules within dentin and enamel [4]. This oxidative reaction breaks the double bonds of the chromophores, which absorb light and give teeth their stained appearance. The resulting smaller, less pigmented compounds lead to the whitening of the tooth. There are some side effects associated with TB, the most significant being tooth sensitivity (TS), which affects 49 to 100% of patients undergoing in-office TB [5; 6; 7]. Hydrogen peroxide and its byproducts, due to their low molecular weight (34.01 g/mol), can penetrate the porous enamel and dentin, reaching the pulp tissue and triggering an inflammatory response. Tooth sensitivity is defined as acute pain triggered by thermal, chemical, mechanical, or osmotic stimuli on exposed dentin, varying in intensity and duration, and not attributable to any other dental pathology [8; 9]. The intensity of this sensitivity is related to the concentration of peroxide in the bleaching system used, its components, and the duration of its application. Discomfort is usually noticed immediately after the bleaching therapy or within the next few days. Generally, the sensitivity is quite tolerable and can disappear completely with proper treatment [10]. Various desensitizing therapies are employed to alleviate pain and improve patient comfort after TB [11; 12; 13]. These treatments include applying chemical desensitizing agents to the sensitive area, which help occlude dental tubules and reduce nerve excitability by depositing ions on the dentin surface and promoting remineralization. Common desensitizing agents include sodium fluoride (NaF), potassium nitrate, and strontium chloride. These agents can be incorporated into denitrices or mouthwashes for at-home use or into gels or varnishes for professional application. Resin sealers, dentin adhesives, and glass ionomers are also used as more permanent solutions to mechanically seal dental tubules. While anti-inflammatory and analgesic drugs have also been recommended as desensitizing agents, a recent systematic review and meta-analysis did not support their use for this purpose [14]. Given the mechanism of hydrogen peroxide, treatments using antioxidant substances have emerged as alternatives to mitigate significant oxidative stress and pulp tissue damage. Several in vitro studies have shown that antioxidant compounds such as alpha-tocopherol (vitamin E) and ascorbic acid (vitamin C) could effectively protect against hydrogen peroxide-induced damage to pulp cells [15]. However, randomized clinical trials (RCTs) are needed to confirm their effectiveness. Additionally, advanced technologies such as laser therapy have been proposed as effective modalities for treating TS [16]. The action of lasers for treating tooth sensitivity can be explained through several mechanisms.