Though I know that vitamin C does not directly chelate mercury, I used to get a weird reaction to it that would bring on further mercury symptoms. Everytime I took it, they'd come up and I gather it was because the mercury in me was more readily available and the vitamin C perhaps helped increase the glutathione levels in my body, which then enabled me to detox a bit easier, hence the increased symptoms. I don't get this now because I'm much sicker and the mercury is not wher eit used to be (repoisoned, and other health issues).

I actually had to lower the dose of vitamin C (back when it would do that to me), or stop and wait for the symptoms to die down a bit and then try again. It did actually help me alot then to push some of the available mercury out more. It doesn't chelate, but obviously if the mercury has already been pulled into available tissues by some other means (for me it was a DMPS injection), then it doesn't take much to encourage further excretion. It was coming out of the pores of my skin also and causing the mercury rash. I didn't need to chelate at that stage because anything would increase the detox very easily. Selenium, vitamin C, vitamin E. Not chelating agents at all, but obviously they do help and encourage/support detox.

However, it's possible that your daughter is reactive with an agent in the vitamin C itself too. Very hard to tell. Do her symptoms appear to be an increase of what she already suffers? If so, then it's possible the vitamin C is actually encouraging some type of detoxing and increasing symptoms. But I would just try very small doses or something and watch what happens. Other than that, I'd just not let her take it again. Try another kind or something if it seems to be an allergy or intolerance.