These people come in to clinic frequently. Occasionally, it's a true bowel obstruction, which is a potential surgical emergency. But most are just plugged up, don't drink enough and eat poorly. It's usually a chronic problem. And opioids, muscle relaxers, antihistamines, and many others definitely make it worse.
6 oz of Mag Citrate, mixed with Ginger ale or 7-Up often does the trick. If it hasn't worked in 4 hours, repeat. Either that or the nurse with the smallest hands has to go mining for nuggets.
If the eagle still has not landed, I often prescribe 1 gallon of Go-Lytely (I love that name). This is the bowel prep prior to a colonoscopy.
It's never once failed. Those poor devils feel like they're crapping food clear back from when Nixon was president.
(not medical advice. Talk to your doctor if you're FOS)