As someone with IBS who's always irregular, this shit is the literal best pretty much no matter what your problem is. Diarrhea? Take some metamucil. Constipation? Take some muthafuckin metamucil. This shit will run through, grab all your poo and consolidate it into a nice, clean moveable mass. Bonus: it tastes kind of like Tang.
Some more interesting things about tea: it is indeed the gentlest way to take caffeine as the compounds in tea slow the absorption of caffeine into the body so you're not getting an immediate hit like with coffee. However, black tea can hurt your digestive system after a while. Personally it irritates my bladder if I drink more than a cup a day or so.
That said, if you want the caffeine and more health benefits, drink a nice HQ green tea instead. Or a NQ one I guess. (I like the Kirkland ones sold at Costco which are a rebranded Japanese sencha/matcha blend). Green tea is chock full of more health benefits than black tea and without some of the downsides of black tea.