Not completely. Every once in a while they accidentally puncture the intestine with the probes and that becomes a significant medial problem. It doesn't happen often, but that is still a risk that doctors need to consider. If you are over 50 getting one every 10 years is a good idea, and there is some consideration if younger might be worth it. However so few people get colon cancer under 40 that it isn't worth the risks for most - but if there are other signs of a problem (either family history or symptoms) that changes things and it may be worth it.