The aim of this randomized, controlled trial is to evaluate the safety and efficacy of Suxiao Jiuxin Pills for prevention of acute mountain sickness (AMS) after high altitude exposure in healthy volunteers, and its influences on physiological indicators before and after high altitude exposure.

Acute altitude illness refers to a group of clinical symptoms that occur when travelers are exposed to high altitude in a short period of time, due to the body's incomplete or dysregulated adaptation to hypoxic environment, which may occur at anytime from a few hours to 5 days after ascending, with severity ranging from mild discomfort to life-threatening conditions such as cerebral and pulmonary edema. AMS is the most common manifestation, characterized by headache, gastrointestinal symptoms, fatigue, weakness, dizziness, light-headedness, etc. Lake Louise Score (LLS) system has been widely used to evaluate AMS, and the version updated in 2018 kept nausea/vomiting, fatigue, lassitude and dizziness as the assessment symptom for AMS, and each symptom can be scored as 0-3 according to severity. AMS can be diagnosed when headache exist and total LLS is ≥ 3 points. Based on total scores, AMS can be defined as mild (3-5), moderate (6-9) and severe (10-12).

Suxiao Jiuxin Pill is a traditional Chinese medicine compound composed of Ligusticum Wallichii and Borneo Camphorwood, and its clinical safety has been confirmed. A total of 168 subjects will be enrolled in this randomized, controlled trial, and eligible subjects will be randomized into Suxiao Jiuxin Pill intervention group or Placebo group after informed consent is obtained. The study drug or placebo will be given orally for 4 days at sea level, arrival day and another 2 days at high altitude. The clinical assessments, including LLS system, sleep questionnaire scores, vital signs, blood oxygen saturation, blood routine test, blood biochemistry test, echocardiogram, Holter, ambulatory blood pressure monitoring, pulse wave velocity, and transcranial Doppler ultrasound parameters will be repeatedly performed at sea level and high altitude.

Inclusion Criteria:

• Healthy volunteers, aged 18-55 years old (including boundary values);
• Female volunteers weighing ≥45.0 kg and male volunteers weighing ≥50.0 kg, with a body mass index (BMI) between 19.0 and 28.0 kg/m2 (including boundary values) (BMI=weight kg/height m2);
• Primary residence at an altitude of <500 meters above sea level;
• Not ascending to an altitude >2500 meters in the 6 months prior to the screening period;
• Voluntary participation with written informed consent.

Exclusion Criteria:

• Subjects with previous severe high altitude sickness such as high altitude heart disease, high altitude pulmonary edema, high altitude cerebral edema, and high altitude erythrocytosis;
• Subjects with established cardiovascular or cerebrovascular disease or uncontrolled hypertension (systolic blood pressure ≥ 140mmHg and/or diastolic blood pressure ≥ 90mmHg at screening);
• Subjects with clinically significant respiratory disease, digestive disease, liver disease, central nervous system disease, psychiatric disease, metabolic disease, renal disease, anemia, or acute infection;
• Subjects with primary headache (migraine, tension headache, cluster headache, etc.) or secondary headache (headache associated with infection, cerebrovascular, etc.) within 1 month prior to screening;
• Subjects with insomnia, anxiety, depression or a history of motion sickness or airplane sickness;
• Subjects with Lake Louise Score ≥ 2 at screening;
• Subjects with left index finger oxygen saturation <95% at screening;
• Subjects with alanine aminotransferase or aspartate aminotransferase > 2 times the upper limit of the normal range, or creatinine > upper limit of the normal range at screening;
• Subjects with hypersensitive C-reactive protein > upper limit of normal range at screening;
• Subjects with history of surgery or blood donation within 3 months prior to screening;
• Subjects with any medication or non-pharmacological intervention (including dietary supplements) to prevent or treat acute mountain sickness within 14 days prior to screening;
• Smoking ≥20 cigarettes per day within 3 months prior to screening;
• Contraindications to the use of Suxiao Jiuxin Pill, such as allergies;
• Women who are pregnant, breastfeeding, or have a positive pregnancy test (human chorionic gonadotropin test);
• Alcohol abuse [more than 28 standard units of alcohol per week (1 standard unit contains 14 g of alcohol, e.g., 360 mL of beer, 45 mL spirit with 40% alcohol by volume, or 150 mL of wine)] or substance abuse within 6 months prior to screening;
• Participation in another interventional clinical study within 3 months prior to screening;
• Other conditions, in the opinion of the investigators, make participation in this study inappropriate.

The participants, care providers, investigators, and outcome assessors will be blinded throughout the clinical trial. To ensure blindness, the experimental drug and placebo will be similar in dosage, odor, and taste. Suxiao Jiuxin Pill and placebo will be repackaged and distributed according to standardized procedures.