One of the most important features that distinguish malignant tumors from benign tumors is that besides growing in the primary site to form primary foci, when the primary foci grow to a certain size, tumor cells will break away from the primary foci and reach other organs of the body to form metastatic foci through blood, lymphatic fluid or natural cavities of human body. Therefore, only in the early stage of tumor formation and before the metastases are formed can the tumor be cured by complete removal of the primary site. Unfortunately, under the current medical condition, for most tumors, they can only be diagnosed by imaging when the number of tumor cells grows to more than 1 million in the body, and early tumors often have no specific clinical symptoms, or even no symptoms at all. If these metastases are not removed, they will continue to grow in the body and eventually lead to tumor recurrence. Chemotherapy has become one of the main treatment tools for middle and advanced stage tumors. Most of the commonly used chemotherapy drugs are absorbed through oral gastrointestinal tract or administered intravenously (rarely through intramuscular injection), which is a systemic treatment compared to local treatment such as surgery and radiotherapy. Chemotherapeutic drugs are cytotoxic drugs that affect cell proliferation by affecting the synthesis of biomolecules (nucleic acids and proteins) in cells. As the saying goes, “medicine is toxic in three parts”, this is because both tumor cells and cells of normal human organs such as liver, heart, kidney and digestive tract will be affected by chemotherapy drugs, only that normal human cells and tumor cells have different sensitivity and tolerance to chemotherapy drugs and different recovery ability after being harmed by chemotherapy drugs. The doctors use these differences to control the drugs in a certain dose, so that one chemotherapy can kill a certain number of tumor cells, but also let the normal cells suffer less damage, or recover the normal proliferation ability earlier than the injured tumor, when the normal cells recover and the tumor cells have not fully recovered, then give the second chemotherapy to let the drugs further kill a certain number of tumor cells, of course, the normal cells will be hurt by the chemotherapy drugs again. But if they can recover earlier than the tumor cells, the doctor will give the third chemotherapy …… so repeatedly until the number of tumor cells in the body is so small that they can be removed by the body’s immune system, then the tumor can be cured, or so small that the residual tumor cells can remain stable and human body for a long time. The purpose of prolonging the patient’s survival is to “peacefully coexist” with the body. Previously we talked about that chemotherapy drugs are actually cytotoxic drugs, theoretically the higher the dose of chemotherapy the more it kills the tumor cells, but the cells of organs such as liver, heart, kidney and digestive tract are also affected by chemotherapy drugs, these organ cells are damaged clinically as liver function, heart function and kidney function, which can have corresponding symptoms and abnormal laboratory test results, and even lead to death in serious cases. Therefore, chemotherapy should not be excessive, because excessive chemotherapy can kill normal cells and tumor cells, and chemotherapy should not be insufficient, especially for some primary tumors. The doses of chemotherapy regimens recommended by various treatment guidelines are mostly obtained from clinical trials. Patients and their families often have doubts about chemotherapy due to insufficient knowledge of chemotherapy and hearsay, and the most common questions we doctors are asked in the clinic are “Is chemotherapy very uncomfortable?” and “Can I tolerate chemotherapy? and “Can I tolerate chemotherapy?”. The most common questions we are asked in the clinic are “Is chemotherapy very hard?” and “Can I tolerate chemotherapy? Prior to the advent of central antiemetic drugs in the 1990s, chemotherapy-induced GI reactions, like severe “early pregnancy reactions,” were indeed daunting and one of the main reasons for poor chemotherapy adherence. However, with the emergence of central antiemetic drugs in the 1990s and the increasing variety of drugs, doctors can predict the emesis of different chemotherapy regimens and use a reasonable combination of neurokinin.1 (NK-1) receptor blockers (not yet available in China), 5-H3 receptor blockers, dexamethasone, metoclopramide, tranquilizers, H-2 receptor blockers/proton pump inhibitors, etc., which can effectively control all types of emesis caused by chemotherapy, including acute emesis, delayed emesis, anticipatory emesis, breakthrough emesis and refractory emesis. All can be effectively controlled. Just as patients did not have to refuse surgery for fear of pain after the advent of anesthetic drugs, patients now do not have to refuse chemotherapy for fear of vomiting. The patient’s ability to tolerate chemotherapy, or in medical terms, the presence or absence of contraindications to chemotherapy, is a medical assessment that every patient routinely undergoes before receiving chemotherapy, and the purpose of this assessment is to ensure the safety of chemotherapy, and it should be repeated every time before chemotherapy. The most important part of this evaluation is the function of important organs. The doctor can obtain the function of important organs of the patient through auxiliary tests such as ECG, left ventricular ejection fraction, liver function, kidney function and laboratory tests, and make a judgment on whether the patient can tolerate chemotherapy. Patients with poor cardiac function should avoid anthracycline anticancer drugs, and patients with poor renal function should be cautious in the application of platinum-based drugs. Although we have no evidence that these organ-protective drugs will affect the efficacy of anti-cancer drugs, there is also no evidence that they will not affect the efficacy of anti-cancer drugs. Therefore, the indications should be strictly controlled when using such organ-protective drugs to avoid mutual interference of drugs, which may reduce the efficacy of chemotherapy drugs and increase the toxic side effects of chemotherapy drugs. For those who have normal organ function tests, no other concomitant diseases, or mild functional abnormalities after chemotherapy, and can recover on their own before the next cycle of chemotherapy, there is no need to use liver-protective, heart-protective and kidney-protective drugs in chemotherapy; while for those who have concomitant diseases, such as hepatitis B, hypertension, diabetes, or drug-related hepatitis and abnormal kidney function tests caused by the previous cycle of chemotherapy, and are estimated to be unable to recover on their own, in order to ensure that the next cycle is carried out on time, some drugs with different effects can be used appropriately. In order to ensure that the next cycle is carried out on time, some adjuvant drugs with different mechanisms of action can be used appropriately to promote the recovery of liver and kidney functions, while the treatment of concomitant diseases should be emphasized; if severe organ function damage is caused by chemotherapy, chemotherapy should be stopped in time, and the selection of such drugs should be avoided again in the future. In conclusion, chemotherapy is a double-edged sword in the treatment of malignant tumors. The formulation of chemotherapy regimen, including drug selection, combination, dose, interval, order of administration, route, timing of chemotherapy, combination with other therapeutic means, as well as the treatment of side effects and organ protection, should be regulated so that this sword can point directly at the tumor, otherwise it will bring fatal damage to patients.