Hello! Thank you for the query. Codeine can cause constipations. Due to constipations stool gets hard and can cause rectal bleeding due to hemorrhoids or mucosa irritating. Such blood is usually bright red and on the surface of the stool. Hemorrhoids are the most common reason of the blood in stool, however such bleeding can be due to other reasons like rectal polyps or colon cancer. If you are 50 or older, the colon cancer is something that should be ruled out. To do that, colonoscopy should be performed. If you are younger, you can try some treatment and if the blood in stool still will be present, than colonoscopy should be done. First of all, you should avoid constipations, have more fiber rich products and mineral water. You should avoid fatty foods and soft drinks. Sitz baths with Quercus Cortex can be helpful as well. You can also try prescription free hemorrhoids suppositories. If such treatment wont give any effect in a week or so, colonoscopy should be done. Hope this will help. Regards.