For oncology patients, the occurrence of diarrhea is generally due to multiple factors, some of which are even causal, so the analysis of etiology and causative factors directly affects the effect of treatment. Here we trace the origin of “diarrhea” in oncology, investigate the mechanism of diarrhea caused by pathogenic causes, and briefly analyze it from the perspective of pathophysiology, so as to follow the vine and prescribe the right medicine to guide the treatment of diarrhea in oncology. The basis of diarrhea pathogenesis is the increase of secretion due to gastrointestinal tract dysfunction or disorder on the one hand, and the decrease of absorption and/or acceleration of dynamics on the other hand, resulting in thin stool and increase of stool frequency, which eventually lead to the occurrence of diarrhea. The quantitative index is the resolution of liquid stools more than 3 times a day or a total amount of more than 200g a day, with a water content of more than 80%. From the pathophysiological point of view, diarrhea is classified into five types: secretory, exudative, osmotic, motile, and malabsorptive. Which conditions of diarrhea will occur in oncology? I. Treatment-related diarrhea (1) Chemotherapy-induced diarrhea (CTID) Diarrhea is a common side effect of chemotherapy. The intestinal mucosa divides and proliferates rapidly and is easily damaged or inhibited by cytotoxic antitumor drugs, resulting in increased secretion and decreased reabsorption, and disruption of fluid balance in the intestine. The occurrence of diarrhea in patients who are receiving chemotherapy or have received chemotherapy within the last 2 weeks should be given sufficient clinical attention. Fluorouracil, methotrexate, cytarabine, doxorubicin, and molecularly targeted agents (Iressa, Erbitux) can cause diarrhea, and the incidence of diarrhea can reach 35-85% with conventional doses of fluorouracil used continuously for 5 d. The risk of diarrhea from irinotecan is even better known and is divided into two types: early-onset and late-onset. Early-onset diarrhea occurs during or shortly after the end of irinotecan titration and is mainly associated with increased excitability of cholinergic nerves, for which anticholinergic symptomatic treatment is effective; late-onset diarrhea usually occurs 24 hours after drug administration and is associated with peak concentrations of intermediate metabolites of CPT-11 (SN-38) in the blood and with poor absorption of water and electrolytes and high secretion of mucin, involving three mechanisms of osmosis, secretion and exudation. (2) Radiotherapy-associated diarrhea Diarrhea can occur in both small bowel and rectal radiotherapy, and among them, diarrhea from radiation proctitis is more common. Radiation proctitis is a common complication of intrapelvic radiotherapy for rectal cancer and cervical cancer. Early acute rectal reaction: the intestinal mucosal epithelium with fast division and short cell cycle is especially sensitive to radioactivity. Radiation causes impaired regeneration of the intestinal surface epithelium, capillary exudation, and affects the barrier function of the mucosa, while the normal absorption function is also affected, leading to watery diarrhea, rectal mucosal edema, intestinal spasm and enhanced intestinal peristalsis within a short period of time; delayed proctitis: radiation can also make vascular endothelial cells to swell and form foam-like changes, leading to ischemic necrosis and further fibrosis, scarring and deformation of the intestinal wall, affecting the advancement of muscle contraction and motility, usually within 1 year or several years after radiotherapy. (3) Surgery: Surgical reasons for removing part of the intestinal canal, resulting in altered intestinal function and reduced absorption area, can cause malabsorptive or motility diarrhea; postoperative patients can have diarrhea due to damaged intestinal mucosal epithelial cells, bacterial flora disorder, antibiotic treatment, acute stress reaction, etc. Second, tumor-related diarrhea Neuroendocrine tumors can produce high concentrations of pro-secretory hormones to promote intestinal mucosal secretion, mainly carcinoid syndrome, gastrinoma, VIP tumor, thyroid medullary tumor; tumor ulceration and comorbidity due to treatment can also make intestinal endocrine secretion increase. Infectious diarrhea The occurrence of myelosuppression caused by chemotherapeutic drugs, the low immune function of tumor patients, the active proliferation of normal intestinal flora, the occurrence of intestinal infection, triggering exudative diarrhea; unclean diet or contact with unclean places, infection with pathogenic microorganisms leading to diarrhea; long-term use of antibiotics due to treatment, resulting in the dysbiosis of intestinal flora, can directly or indirectly lead to the occurrence of diarrhea. What are the clues to catch diarrhea in oncology? (1) For patients with diarrhea, we should not only record the characteristics of diarrhea (duration, number of bowel movements, frequency, stool properties), chemotherapy drug administration and diet in detail, but also evaluate the accompanying symptoms and signs such as fever, urgency, abdominal pain, emaciation and severe water loss. (2) To assess the accompanying symptoms and signs of fluid loss (2) To identify the specific causes and mechanisms of diarrhea with the help of laboratory and adjuvant examinations, mainly focusing on stool examination, blood picture, renal function, water, electrolytes, acid-base and endoscopy. In conclusion, it is important to be aware of the causes of diarrhea in oncology, to achieve early detection, comprehensive assessment and early treatment, and, in particular, to identify those patients who are at high risk of developing severe diarrhea.