The actors, who are the co-chairmen of Welsh football team Wrexham FC, teamed up with awareness organisation Lead From Behind to share details of their procedures and diagnoses.
In a video posted on social media, Reynolds says that he made a bet that McElhenney could not learn to speak the Welsh language, or he would film and broadcast himself undergoing the intimate procedure.
After explaining the bet in Welsh language, McElhenney smiles, while Reynolds adds: “Rob and I turned 45 this year which means getting to an age is getting a colonoscopy.
“It’s a simple step that could literally, and I mean literally, save your life.”
The video then shows Reynolds arriving at the hospital and undergoing the procedure.
He is told by a doctor that an “extremely subtle polyp” has been found on his colon.
“This was potentially life-saving for you. I’m not kidding. I’m not being overly dramatic,” the doctor later tells him.
“You are interrupting the natural history of a disease, of a process, that could have ended up developing into cancer and causing all sorts of problems.”
“You reached the age of 45, you had a routine screening and there you go, that’s why people need to do this. This saves lives, pure and simple.”
McElhenney then reveals that he also underwent the procedure, and that it was also filmed, joking that he “could not go wrong” in terms of comparing his results to Reynolds’.
“They either find nothing, and that means my colon is cleaner than his, or they find a polyp and it’s either bigger than his… or it’s smaller than his which means I have less of an opportunity to have cancer,” he says.
“Either way I win.”
McElhenney is later told that three polyps were discovered on his colon, which were also removed.
The video ends with a message telling people to ask their doctors about colonoscopies, and “helping Rob and Ryan kick colon cancer’s ass”.