This experience is common for many different supplements—like multis and vitamin C. Here’s one reason that may happen with the latter, as well as one A+ vitamin C , option for those with sensitive stomachs. “This is usually because people are consuming the acidic form of vitamin C—ascorbic acid—and for some, their stomach can’t handle the extra acid,” Michels once told mbg. To be clear, we are talking about doses upwards of 500 milligrams of vitamin C in one sitting, over five times the daily recommended dietary allowance for adults. Of course, personal differences in physiology as well as stomach sensitivity and the setting in which the supplement are taken all play a role as well. But it’s a pretty common short-term side effect of a large load of ascorbic acid, and it’s largely dependent on the dose and form of vitamin C you’re taking. In short: If you’ve experienced stomach aches with your vitamin C supplement, you may want to consider opting for a gentle, bioavailable form like the one in mbg’s vitamin C potency+.