Fluorouracil (5-FU) has been one of the most essential drugs in gastric cancer chemotherapy, and it is impossible for any gastric cancer chemotherapy patient to leave fluorouracil-based drugs. Fluorouracil drugs were the first drugs to enter gastric cancer chemotherapy and have been used for more than 50 years. In August 2009, the oral fluorouracil drug (Aiswan S-1) from Roc, Japan was launched in China, providing a good choice for gastric cancer patients. As a new generation of oral chemotherapy drug, Aiswan (S-1) has a better safety profile and is easy to take. Patients can use it at home, which reduces the need to travel to and from the hospital and has very little psychological and family impact on patients. S-1 is an oral fluorouracil derivative, which consists of tegafur, gemini and potassium octreotide in a molar ratio of 1:0.4:1, and exerts anti-tumor effects by converting into 5-FU in the body. Gimepyrimidine potently inhibits dihydropyrimidine dehydrogenase (DPD), a 5-FU catabolic enzyme, to help maintain effective concentrations of 5-FU in blood and tumor for a long time, thus achieving the same efficacy as 5-FU. The same efficacy as continuous intravenous infusion, significantly increasing its antitumor effect. Otilacid potassium is mainly distributed in the gastrointestinal tract, inhibiting orotate phosphoribosyltransferase (OPRT) and blocking the phosphorylation of 5-FU, thus effectively reducing its gastrointestinal toxicity. Since 1999, Estevan has been approved in Japan for the treatment of advanced gastric cancer, followed by seven other indications for head and neck tumors, colorectal cancer, and non-small cell lung cancer. In addition, the published ACTS-GC study (New England Journal of Medicine, 2007) demonstrated that adjuvant chemotherapy with S-1 alone can achieve an overall survival of 80.1% at 3 years after gastric cancer surgery, a 10% increase compared to surgery alone, demonstrating that S-1 alone is safe and effective when used as adjuvant chemotherapy after radical gastric cancer surgery. The results of the SC101 study (ASCO, 2008), a marketed clinical trial of S-1 in Chinese gastric cancer patients, confirmed that S-1 can be used alone or in combination with cisplatin for the treatment of advanced gastric cancer, with an overall survival of 14.4 months, exceeding one year, which is the longest overall survival achieved by chemotherapy alone in the published literature worldwide. Overall survival data. Patients in the Aiswan (S-1) and Aiswan® (S-1) combined with cisplatin groups were well tolerated. The efficacy and safety of both Aiswan (S-1) alone and Aiswan (S-1) in combination with cisplatin regimens were confirmed in Chinese gastric cancer patients. Aiswan (S-1) combined with cisplatin regimen is expected to become one of the standard treatment regimens for advanced gastric cancer in China. Therefore, as an oral 5-fluorouracil drug, S-1 has the following three main features: 1) high efficiency: the SC-101 clinical trial in China achieved a long median overall survival of 14.4 months for advanced gastric cancer; 2) convenient: oral chemotherapy; 3) safe: it contains DPD enzyme inhibitors and gastrointestinal mucosal protective agents, and has been proven in clinical trials to have mild toxic side effects 3. Safety: contains DPD enzyme inhibitors as well as gastrointestinal mucosal protective agents. The dose is 40mg/m twice a day based on the body surface area and is taken orally half an hour after breakfast and dinner. S-1 is currently available in two sizes, 20mg and 25mg, to facilitate different doses for patients. Acevan (S-1) is usually recommended to be taken orally for four weeks with two weeks off or two weeks with one week off, mainly due to the mechanism of action of 5-FU. It can also be used in combination with other oncology agents to form chemotherapy regimens, such as SP regimen (esvan in combination with cisplatin) SOX regimen (esvan in combination with oxaliplatin), etc. During the course of use, Estwan (S-1) patients should undergo hematological examination every two weeks and keep a record of their dosing process so that the doctor can better adjust the dose and reduce the chance of side effects.