An actual diagnosis of constipation has a 3 month standard. That is, you have to be having fewer than 3 bowel movements a week OR hard stools, straining on defecation or retained feces (you poop, but not completely) 25% of the time for three months.
Most nurses and doctors don’t wait nearly that long before they recommend at least dietary changes, increased water and exercise, bulk laxatives, and stool softeners, though.
It also depends on the age of the patient, any co-morbidities, and how they feel about it. If someone is in a lot of pain, I’m going to intervene a little sooner than someone who just doesn’t poop all that frequently but isn’t bothered by it. If I have a patient with a history of diverticulitis and/or bowel perforation, I’m going to get concerned sooner.
It also depends on what their particular pattern is. The books say people should be pooping at least twice a week, but our bodies don’t read the book. My daughter had one bowel movement a week for her first year and a half. It was on Fridays at 1:30. Very considerate of her, but most importantly, that was her specific pattern, she wasn’t in pain and she didn’t have to strain when she did poop, so her pediatrician was completely unconcerned.
As people get older, they tend to decrease in defecation frequency. This is due to a combination of factors, including eating less and moving less, but again you want to find out what their normal is before you get too concerned.
A sudden severe pain, a swollen belly, a rigid, boardlike abdomen or black stools are emergency call symptoms. Those may indicate a bowel rupture and bleeding. They may happen after someone hasn’t had a bowel movement in a few days, OR they can happen with only a partial blockage, to someone who has been having diarrhea or flat ribbony stools - some fecal matter is passing the blockage or area of constriction, but there’s a boulder of poop partially blocking the pipes.
Miralax is polyethylene glycol, which sold under a different name(s), is part of the standard bowel prep before a colonoscopy. Of course, the dose for that is a little higher. But yes, it would be expected to at least give you some loose stools, if not outright diarrhea, with or without anal leakage. Fun times.