Background: The effects and biological mechanisms of auricular acupuncture on the human body have been increasingly observed in clinical and experimental studies, particularly the analgesic effects. Auricular acupuncture in the Shenmen, Nervous Subcortex, Jaw and Tooth groups has been shown in the Acupuncture 2 textbook of the Faculty of Traditional Medicine - University of Medicine and Pharmacy, Ho Chi Minh City and clinical studies to be effective in reducing facial pain.

This study aims to examine: (1) the change in facial pain threshold when performing auricular acupuncture in the acupoints of Shenmen (TF4), Nervous Subcortex (AT4), Jaw (LO3) and Tooth (LO1) on the left ear, (2) the change in facial pain threshold when performing auricular acupuncture in the acupoints of Shenmen, Nervous Subcortex, Jaw and Tooth on the right ear and (3) examining the unfavorable effects of the procedure.

The effects and biological mechanisms of auricular acupuncture on the human body have been increasingly observed in clinical and experimental studies, particularly the analgesic effects. When the downward neurotransmitter pathway is activated, endogenous opioids (beta endorphins) are released, which inhibit pain perception. Furthermore, according to the gate control theory (spinal segmentation mechanism), auricular acupuncture aids in the activation of pain-suppressive stimuli by myelinated afferent fibers (Aβ), as opposed to stimuli with damage from poorly myelinated (Aδ) or unmyelinated (C) fibers. Besides, according to human physiology, the trigeminal nerve is involved in sensory control of the face, has a branching distribution to the skin of the pinna, and has afferent conduction of mechanical stimuli acting on it. Auricular acupuncture in the Shenmen, Nervous Subcortex, Jaw and Tooth groups has been shown in the Acupuncture 2 textbook of the Faculty of Traditional Medicine - University of Medicine and Pharmacy, Ho Chi Minh City and clinical studies to be effective in reducing facial pain.

This was the first study in Vietnam to investigate the effect of auricular acupuncture on the change in facial pain threshold during auricular acupuncture on acupoints such as Shenmen, Nervous Subcortex, Jaw and Tooth. The findings of the study can be used to supplement scientific evidence regarding the effects of auricular acupuncture in the treatment of clinical facial pain.

The investigators performed a randomized, controlled crossover experiments. A crossover study has two advantages over both a parallel study and a non-crossover longitudinal study. First, the influence of confounding covariates is reduced because each crossover patient serves as their own control. In a randomized non-crossover study it is often the case that different treatment-groups are found to be unbalanced on some covariates. In a controlled, randomized crossover designs, such imbalances are implausible (unless covariates were to change systematically during the study). Second, optimal crossover designs are statistically efficient, and so require fewer subjects than do non-crossover designs (even other repeated measures designs). In our study, the subjects are randomly assigned to two arms of the study which receive different treatments. A crossover trial has a repeated measures design in which each patient is assigned to a sequence of two treatments, one of which is auricular acupuncture, and a placebo test is performed on that volunteer a week later. Wilcoxon sign-rank test, paired T test and Pearson's chi-squared test were used for comparison of means.

Inclusion Criteria:

• Men and women between the ages of 18 and 29 who are in good health.
• BMI ranging from 18.5 to 22.9 kg/m2 (classification of IDI & WPRO for Asians).
• Voluntarily consented to participate in the study, read, was thoroughly explained, and signed the consent form to participate in the study.
• Mental and physical well-being were assessed throughout the study period (on the Depression Anxiety and Stress Scale of 21 with a stress score of 15 points).
• Vital signs are within normal ranges:

Pulse: 60-100 pulses per minute Systolic blood pressure: 90 to 139 mmHg. Diastolic blood pressure: 60 to 89 mmHg. Temperature: 36.3-37.1 degrees Celsius. At rest, the breathing rate is 16 3 times per minute, and the SpO2 level is 92%.

- Volunteers have no other chronic medical conditions: Thyroid disease, autonomic neuropathy, hypertension, diabetes, respiratory diseases (asthma, pneumonia, chronic obstructive pulmonary disease, etc.) etc) by asking for medical history and medical history.

Exclusion Criteria:

• A feeling of fear or anxiety appeared immediately before the test (rated on the Depression Anxiety and Stress Scale of 21 with stress scores greater than or equal to 15 points).
• Use of stimulants such as alcohol, beer, coffee, and tobacco within 24 hours before the study.
• The auricular skin was examined with lesions (scars, lacerations, scratches, bites).

Demographic characteristic and outcome data will be shared