## 1 Introduction  
Over the past decade, there is an upward trend among the general public to enhance the dentofacial esthetics. This has been warranted by an increased number of patients seeking esthetic dental procedures such as orthodontic treatment and tooth whitening [1, 2]. People have placed high value on tooth color, and previous studies have suggested that many of them are dissatisfied and want them to be improved [2, 3]. Furthermore, people judge a person's social competence, intellectual prowess, psychosocial adjustment, and life satisfaction more favorably when their teeth are white [3]. Therefore, tooth whitening has become one of the most frequently sought-after dental treatment in recent years [4]. Subsequently, it has renewed the dental profession's interest in esthetic dental treatment due to an individual's desire for whiter and brighter teeth. Currently, four tooth-whitening modalities are available: professionally applied (in-office), dentist-supervised (at-home), nonsupervised consumer-purchased/over-the-counter (OTC), and do-it-yourself tooth whitening (DOY). However, tooth whitening depends on several factors such as the method of application, pH of the whitening agents, fluctuation of irradiation, length of photoactivation, tooth size, and the selective absorption of the wavelength of the irradiation complex procedure [5, 6]. With the increase in demand for over-the-counter (OTC) products, manufacturers have developed whitening productsin the form of rinses, chewing gums, gels, toothpaste, strips, paint-on films, and whitening pens. These products are readily available in the market without prescription, and are also easy to use [7]. The lack of data and ease of access to these products is concerning, as they may harm customers' dentition. Over-the-counter products are vulnerable to mishandling, overuse, and abuse due to their 'do-it-yourself' nature [8]. Citric acid is considered as one of the natural products for tooth whitening and many of the tooth-whitening products contain citric acid in low concentrations to avoid enamel erosion [9; 10]. Although the availability of OTC whitening products has increased their popularity, their adverse effects and safety remain a concern due to the possibility of damaging dental tissues [11; 12; 13]. In addition, it is worth mentioning that the Food and Drug Administration (FDA) does not regulate the use of all tooth-whitening OTC products, and these products do also require the American Dental Association (ADA) seal of acceptance indicating that they are safe and effective when used as directed [14; 15]. Tooth-whitening products usually contain carbamide peroxide (CP) or hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) in different concentrations (CP-10-20%, H\({}_{2}\)O\({}_{2}\)-30-35%) [12]. The active bleaching agent, on the other hand, is H\({}_{2}\)O\({}_{2}\), and its ionization releases free oxygen ions, which cause oxidation of discolored organic pigments in dental hard tissues, resulting in tooth whitening [3]. There is inconsistency in the literature when reporting the adverse effects of tooth-whitening agents (10-35%) in terms of enamel surface morphology alterations and microhardness changes. Several studies have demonstrated that whitening agents negatively affected the enamel surface morphology and microhardness [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27], whereas a few studies using the same concentration of whitening agents showed no such adverse effects [28; 29; 30; 31]. Previous studies on the effect of in-office or at-home bleaching agents on enamel surface changes are abundant and generally satisfying [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. In contrast, evidence on the effects of over-the-counter whitening products on enamel surface morphology alterations and microhardness is lacking.