It depends both on your gut and the skill of your physician, I suspect. Mostly, it just feels a little weird, but there’s discomfort as the front of the tube goes around curves. The first time I did it, there were a few seconds when I gripped the rails along the bed pretty hard and clenched my teeth, but it was only a few seconds. The second time (I’ve only done it twice) I was chatting with the physician, and barely noticed until I suddenly said “ouch” and he apologized, saying he’d just rounded a curve, but the pain was basically gone by the time he apologized. They shove the tube all the way up, and do the exam as they slowly pull it out, and I didn’t find the exam part uncomfortable at all, and enjoyed the guided tour of my innards. (The second doctor gave a guided tour. The first one only spoke when I spoke to him.) I suppose if you were apt to get anxious when the doctor spends a lot of time examining a small bump, it might be disconcerting. (just a small diverticula, not a polyp, in my case.)
I think the pain is more like an abdominal cramp than anything else I can think of, but those last for a while, and this is very brief. Literally a few seconds here and there.
My brother, who had an unexpected unsedated colonoscopy (he had a can of diet soda on the way into the procedure, and they said the colonoscopy was safe, but sedating him wasn’t. He decided he didn’t want to re-do the prep) complained that his colon is very sensitive and he felt every little bump. He was angry that they hadn’t warned him not to drink the soda. I have a friend who had to do one unsedated for some medical reason, and she was appalled when she farted in the doctor’s face. I assured her that 100% of his patients did that, but she felt that it was different because she was awake.
A friend that I talked into trying it unsedated said “I guess my colon isn’t very kinky, it was easy”. On the other hand, if you wake up from colonoscopies feeling painfully bloated, I assume that’s worse during the exam. The “this part of my intestine has been inflated with air/CO2” didn’t bother me at all, either during or after. I also rarely have pain from gas (I just fart if I eat gassy foods, I don’t usually feel discomfort.) YMMV. the newer procedure is to use CO2 instead of air, which can be absorbed by the body, so there’s less distension and little farting after the procedure. (and none during) That may not have caught on everywhere, yet.
Women have kinkier colons then men, because of the uterus and stuff in the same general area. (Trans men should inform their doctor that they have or used to have a uterus prior to a colonoscopy.)
In many countries it’s standard to do it without sedation, and like an MRI they only use sedation if they think there might be a problem for the patient. It’s WAY less painful that having an IUD inserted, which is routinely done unsedated in the US. WAY WAY less painful. I can only assume that men’s pain matters more than women’s.
All this is for a colonoscopy. I’ve also had unsedated upper GI scopes, and that is incredibly uncomfortable because it triggers your gag reflex. I have had issues with sedation, but I would recommend you get sedated for an upper scope (if you aren’t me.)
I know one guy who said he asked for an unsedated colonoscopy and his doctor said “no, there’s too much risk you will move and I’ll perforate your bowel”. I’ve read studies that found the risk of bowel perforation is actually higher with sedation (because the patient doesn’t say “ouch”) and there isn’t a reflex to move suddenly when you feel something uncomfortable in the gut. So I’m pretty sure that’s completely bogus. But there are risks with any procedure, and you don’t want to pressure a doctor into doing something they aren’t comfortable doing.