Go for it! Today is Master-Cleanse day 5 and I started the cleanse on the second day of my period (heaviest) and it was the shortest period I've had in years (4 vs 7)! So I certainly didn't experience any negatives by doig the MC, and actually had the positive effect of a quicker end to it all. Based on everyone else's posts, fatigue is pretty common during the first few days on the Master-Cleanse so it paired up with my period just fine. I figured that if I was going to be down anyway, may as well do something to benefit the rest of my body!