The most common first symptom of rectal cancer is bleeding during bowel movements. Regardless of any rectal bleeding at any time, even if the patient is known to have hemorrhoids or diverticula, doctors should consider the possibility of cancer. Patients with rectal cancer may experience abdominal pain during defecation and a feeling of incomplete defecation. Then what are the complications of rectal cancer? 1.Colonic obstruction is one of the late complications of rectal cancer, which can occur suddenly or gradually. It is mostly caused by tumor proliferation obstructing intestinal cavity or narrowing of intestinal cavity, and can also be caused by acute inflammation, congestion, edema and bleeding at the tumor. There are two kinds of perforation in rectal cancer: perforation occurs locally in the cancer; perforation of the proximal colon, which is a complication of cancer obstruction. After perforation occurs, the clinical manifestation can be diffuse peritonitis, restricted peritonitis or local abscess formation. Diffuse peritonitis is often accompanied by toxic shock, and the death rate is extremely high. 3.Acute hemorrhage is a rare complication of colorectal cancer. It causes a series of symptoms such as increased heart rate, decreased blood pressure, liver cold, decreased urine volume and even shock, which are often life-threatening. Because the lesion of colorectal cancer is closer to the anus, the blood color is mostly bright red or dark red, and often the blood and stool are separated. Only when the amount of bleeding is high, the stool is brownish red and jam-like. The prognosis of rectal cancer is not related to the patient’s gender and age, but is closely related to the course of the disease, the extent of cancer infiltration, the degree of differentiation and the presence or absence of metastasis.