There are complications with any medical procedure. However, colonoscopies are safe and have minimal risk when performed by gastroenterologists, who have been specially trained and are experienced in these procedures.
One rare but serious possible complication is perforation, in which a tear through the colon wall may allow leakage of intestinal fluids. It can be managed with antibiotics and intravenous fluids, although surgery may be required. Bleeding may occur from the site of biopsy or colon polyp removal. It is usually minor and stops on its own. Rarely, transfusions or surgery may be required.
Localized irritation of the vein can occur at the site of medication injection. A tender lump develops, which may remain for several weeks to several months but goes away eventually. Other risks include drug reactions and complications from unrelated diseases such as heart attack or stroke. Death is extremely rare but remains a remote possibility.
In short, complications from these procedures are extremely rare, but they can happen. Your gastroenterologist can tell you more about colonoscopy risks.