Boric acid is borax and suphuric acid mixed; I would not take it iternally!
To avoid nausea take borax in capsules and ascorbic acid as capsules or powder at the same time.
I found water with borax quite sickening myself so I used to put a drop of lemon essential oil in it, but now I dont bother to take it in water anymore; capsules are much more convenient.
A case I have (boy 10 years) take a borax tea every night; that is his daily dosage of borax, taken in his evening tea. Juice is also an option; but it is wise to dissolve the borax in a tablespoon of hot water first.