Hmmm, that is very weird. Do you know which of those was responsible Mega? the Vitamin C, magnesium or niacin? Have you thought of trying each individually to check for a response? Because it is a wise idea to take plenty of antioxidants and minerals when you have mercury. Though I know that at first they can definitely cause reactions sometimes, due to increase in detox. I would hazard a guess and think maybe it was the niacin, but I couldn't be certain.

Fantastic though about the cilantro! And you're not taking much else? E.g. no binding agent like algin or anything?

Sittingduck, true if mercury isn't a part of my problems, cilantro may not be effective, but I always think if it's capable of doing that with mercury, it may also have "some" effect on any other toxins or any other metals. Hard to say, but worth another try in my opinion, as it least it doesn't feed yeast as some chelators can do.