Lugol chromoendoscopy is essential in detecting and characterizing squamous cell carcinoma of esophagus. Esophageal irritating symptom has been reported as an common adverse event, which could be alleviated by sodium thiosulfate solutions. However, sodium thiosulfate was not widely available. N-acetylcysteine has been previously used as a mucolytic reagent, we propose that it may also be used to alleviate irritating symptoms caused by Lugo's chromoendoscopy. A prospective, non-inferior, randomized double-blind clinical trial was designed to study the efficacy of N-acetylcysteine solutions in relieving esophageal discomfort .

Inclusion Criteria:

• Patients at risk of esophageal squamous cell cancer including those over 50 years old, or over 45 years old with family history of esophageal squamous cell cancer;
• Patients with suspected esophageal lesions by screening endoscopy;
• Patients requiring surveillance after endoscopic resection or radiotherapy for esophageal squamous cell cancer one year later。

Exclusion Criteria:

• Patients allergic to iodine or with hyperthyroidism;
• Patients with advanced esophageal cancer, esophageal varices, esophageal ulcer or other conditions inadvisable for Lugol chromoendoscopy;
• Patients with severe gastroesophageal reflux disease or reflux symptoms which may interfered with the outcome measures of current study;
• Patients with postoperative esophageal stenosis affecting endoscopic observation;
• Patients with serious comorbidities that cannot tolerate un-sedated gastroscopy;
• Patients who could not cooperate with the observation including patients with mental disorders, severe neurosis or dysgnosia;
• Patients undergoing sedation or general anesthesia during endoscopy.
• Patients who do not agree to sign informed consent forms or follow the trial requirement.