## 1 Introduction  
The subjective sensation of dry mouth is known as xerostomia, but when this is objectively measured through sialometry, as a decrease in the rates of salivary flow below 0.1-0.2 mL/min of total saliva at rest, and below 0.4-0.7 mL/min of total stimulated saliva, it is known as hyposialia or salivary hyposecretion [1, 2]. The persistence over time of low rates of salivary secretion leads to changes in the oral environment and condition the appearance of lesions on the hard or soft tissues of the mouth [3]. The oral clinical manifestations that appear in these patients are caries, periodontal disease, infections, and talking, chewing, or swallowing difficulties. In addition, a decrease in the sense of taste may occur [3]. Despite the description of various alternatives for the treatment of xerostomia in the last few years, the therapeutic options are not completely satisfactory [2, 3, 4, 5, 6, 7]. The treatment proposals have been mainly based on (a) general hydration measures, avoiding toxic habits, or oral hygiene; (b) saliva substitutes such as sprays, rinses, and gels based on muchic or methylcellulose with a neutral pH and electrolytes, as these provide temporary symptomatic relief and require frequent use; (c) sialogogues such as pilocarpine, which is an effective muscarinic agonist in xerostomia, but can cause significant adverse effects (sweating, diarrhea, bronchospasm, etc.). Cevimeline is also a muscarinic agonist, with better tolerance and lower discontinuation rates. In cancer patients, strategies such as Moderate-Intensity Radiation Therapy (MIRT), the use of amifostine, and salivary gland transfer have been applied to prevent radiation-induced xerostomia. However, surgical interventions have been progressively abandoned due to their complications and limited long-term efficacy [8]. A significant challenge for many clinicians is the recovery of salivary flow. As an alternative to drugs, Transcutaneous Electrical Nerve Stimulation (TENS) has been suggested. This is a conservative technique, less invasive and cheaper, with little or no adverse effects [9; 10; 11; 12]. Although the mechanism of action of TENS on the parotid glands is not completely understood, it is believed that it could stimulate the auriculotemporal nerve, which carries parasympathetic fibers to the parotid gland. This stimulation may enhance salivary secretion by modulating autonomic nerve activity and increasing acetylcholine release, which acts on muscarinic receptors (M3) in the salivary gland, promoting fluid secretion. This offers a valuable option for managing hyposvalication, especially in patients who do not tolerate or are not candidates for pharmacological treatments [11; 12; 13]. This technique has been shown to be safe and potentially effective in stimulating salivary flow in patients with hypofunction of the salivary glands due to conditions such as Sjogren's syndrome, head and neck radiotherapy, or medication side effects [14; 15]. As compared with saliva substitutes such as gels or rinses, TENS has the advantage of inducing the natural production of saliva with minimal secondary effects and without chemical toxicity, and it is easy to use as well [10; 11; 12]. Nevertheless, despite its benefits, there is lack of consensus in the literature about the optimal protocols for the application of TENS, including the frequency, intensity, duration, and location of the electrodes, as well as its long-term effectiveness [10; 11; 12; 13; 14; 15; 16]. This underlines the need to conduct additional studies that assess the safety, efficacy, and standardized TENS protocols in the specific population [12]. The lack of research in this field led us to propose a study to assess the viability and effectiveness of TENS in the treatment of xerostomia. The null hypothesis is that TENS applied to the parotid glands is not able to stimulate their salivary function.