I could never accept my Drs diagnosis of IBS either and about 4 years ago had a sigmoidoscopy that was not pleasant but paled into insignificance compared to colonoscopy! I then was sure the atttacks of bad pain were appendicitis and after second visit to A&E I had emergency surgery incase it was appendix so had large abdominal operation with 5inch scar to check all my organs and guess what nothing wrong this op was followed up 4 months later by the horrible colonoscopy so they had checked out everything and once again perfectly normal.
In one sense having the tests did reassure me but going through them was not something I would wish on anyone, especially the prep for the colonoscopy
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I have know a few people who have had bowel cancer and now know that they either pass blood/black stools or have sudden severe symptons that means hospital visit.