For a medical reason, my doctor advised me to use Epsom salts (magnesium sulfate) several times a day.

So I'm about to head out on a flight this morning and need to take a bag of white crystalline powder.

I'm wondering where is the best place to carry the Espom salts -
a) In my carryon, I can get a residue test and argue with someone that the salts are medically necessary.
b) in my checked luggage, where it is less likely to be scrutinized, but I can't argue if someone confiscates it.

Any suggestions? Normally I would buy when I get there, but we are headed to the middle of nowhere and I have already been warned that I won't be able to buy anything at the store.