Can using laxatives cause internal bleeding?

Unlikely: This is not a common side effect of laxatives, unless your laxative also contains aspirin or an NSAID like ibuprofen. You may have some rectal bleeding from chronic constipation such as a hemorrhoid or an anal fissure which will show itself when you finally go to the bathroom with the laxative though. Let your doctor know about any change in bowel habits you experience.