Rare. Unless severe bleeding.
Not common. If hemorrhoidal bleeding is so severe that it causes anemia, a more detailed work-up should be performed. It would be important to discuss with a doctor or hemorrhoidal specialist such as a colorectal surgeon or general surgeon in order to both create a plan to treat the hemorrhoids and to ensure that the hemorrhoids are actually the source of anemia and not bleeding elsewhere.
Not Common. Although anemia from bleeding hemorrhoids certainly does occur, it is relatively rare. A chart review study from 1976 through 1990 at the mayo clinic and olmsted community hospital in minnesota showed the incidence of this was about 1 patient out of 200, 000 population per year. Most of these patients described squirting blood or passing clots. A colonoscopy can exclude other causes of bleeding.