[Lycaryth]

It will be the powder form in its original container.

I see no reason why they should refuse it, if it was an illegal substance in a major part of the world it wouldn't be freely available to purchase online. I live in Belgium myself and I'm pretty sure the only thing illegal around here would be drugs.
If you worry a lot you could always bring enough for your trip (+a few extra doses) and leave the rest at home in an airtight container. But what you do take with you is best carried in the original container.

Enjoy your trip!!

That being said my mom just got busted for trying to bring Maple Syrup and Cheez Wiz in her carry-on luggage when she traveled from Vancouver to Sydney hahaha. I guess Cheez Whiz is considered a liquid? Bleh.

Candles seem to trip up the scanners too. Always so much fun to have to dig out that one thing that is wrapped tightly in the bottom of your over stuffed carry-on bag

Oddly enough... frozen store-bought ice packs don't seem to show up! Carried one without thinking from Belgium to America once (to keep my Belgian chocolate cool!), I would have thought it would have melted by the time it went through security again in the US but I guess not.