## 4 Discussion
### Comparison between In-Office and At-Home Dental Bleaching
Still compared two at-home techniques using bleaching gels (10% versus 37% CP). Both were used for 3 weeks. The first group (CP 10%) used it for 4 h a day, and the second group (CP 37%) used it for 30 min a day. The result was the same. Therefore, the authors suggest using bleaching gel containing a higher percentage of CP but took less time to use [16]. The most widely used protocol for at-home bleaching is described by [PERSON]. He used low concentrations of CP (10%) overnight [49]. The same author in another study compared the same concentration of CP (10%) for 14 and 21 days and he deduced that 3 weeks is the best duration to obtain good bleaching [50]. [PERSON] et al. showed in-office bleaching mode. In his study, he compared a concentration of HP (6%) with an application of 36 min for a session versus 3 applications of 12 min for a session. The authors proved the moderate efficacy and similar TS [51]. [PERSON], in his study, suggested that the efficacy of treatment is affected by tooth decalcification and the trayless system type, while the trayless system influenced the duration of the treatment [52]. [PERSON]'s used 40% HP for 3 applications every 20 min in a single session and showed the significant efficacy of the bleaching over the 6 month follow-up period [52]. [PERSON] used a purple LED without any bleaching gel and he obtained a good long-term result (with a follow-up of 6 to 12 months) [53]. [PERSON] showed how in-office bleaching using 37.5% HP in two sessions of 8 min  
Figure 5: (**a**) Dental bleaching in dental clinic with a liquid dam is an advantage to avoid gingival tissue irritation. (**b**) Before the treatment, and (**c**) after the treatment. Figure 6: Clinical case of bleaching before and after the treatment. allowed the same degree of bleaching with less TS than 3 sessions of 8 min [54]. [PERSON] proposed the combination of both techniques: 35% HP in office for 45 min and 10% CP for 1 h at-home, reducing the time needed for satisfactory treatment but increasing TS [55]. In [PERSON] work, a hybrid treatment is proposed using 35% HP with different modes of use (between at-home and in-office bleaching) with effective treatment and less TS [56; 57]. In conclusion, both bleaching techniques are effective. Specifically, in-office-bleaching uses a high concentration of molecules (HP and CP) obtaining immediate treatment but with high TS. On the other hand, at-home-bleaching uses low concentrations obtaining lower TS but equal or even higher bleaching efficacy [58]. The prolonged treatment can cause gingival irritation due to the over-edge of the gel over trays; this is difficult to obtain in-office bleaching with the use of a liquid dam [59; 60]. [PERSON] and [PERSON], in their experimental studies, did not find a significant difference in efficacy between in-office or at-home bleaching [61]. In addition, some authors combine the two techniques to determine higher efficacy and lower TS [62]. It is recommended to assess the patient's needs and choose the best treatment for each patient, avoiding the worsening of the tooth and soft tissue health due to the side effects of bleaching gels [63] (Table 6).