I just had this test last month. I took a pill daily for 5 days and was xrayed on day 6. I've heard others say they were xrayed several times. Since I only go to the bathroom if I take laxatives, all the markers were still there. I'm sure if you explain what happened, they'll let you repeat the test if your markers are gone.
I also had the defogram last month...a little humiliating, but not nearly as bad as I thought! They'll xray you, then give you a tube of something to insert in your vagina, and then they'll insert a tube of something in your rear end (sort of a paste) and have you sit on a seat that resembles a toilet seat. My seat had a curtain around most of it, so it wasn't too bad. The radiologist then came and stood beside me and told me when to bare down and when to hold it...I could actually watch the "goo" coming on on a screen in front of me)...it took less than a minute after getting on the "seat". It's pretty embarrassing, but doesn't last long at all, and besides, I figured I'd never see those people again!!