Three days ago I took 200 mg of the Brilliant Blue FCF food dye powder that I bought. I took it in two divided doses of 100 mg, dissolved in 200 ml of water. The first 100 mg I took at 2 pm in the afternoon, and the next 100 mg I took at 6 pm.
There were no noticeable effects from the first 100 mg in the hours immediately after taking it, but a couple of hours after the second 100 mg, I noticed I started to feel angry and irritable (which are symptoms that I have experienced as side effects from several supplements in the past). This anger side effect lasted for around 24 hours.
The next day I noticed that my stool was an intense deep green color, which looked rather odd. So obviously 200 mg is quite a high dose, as you would not normally expect a food dye used in the food or drink you consume to color your stools in this way.
In the next few days after this single 200 mg dose I found that my IBS-D symptoms were noticeably worsened, so I guess the Brilliant Blue FCF may be mildly irritating to the gut (although my intestines are pretty sensitive, because even spicy food will greatly worsen my IBS-D).
I did not notice any improvements in ME/CFS symptoms from the Brilliant Blue FCF, although I only took that one dose.
Because Brilliant Blue FCF seems to worsen my IBS-D (which then makes me feel generally worse) and causes these anger and irritability side effects, I am not going to take it again in these high doses, but I may at some point experiment with much lower doses, eg 20 mg daily.