I took for a test 6 capsules filled borax (00) daily for 10 days. A 00 capsule is approx 800 mg each.
That would be a daily amount of 528 mg of boron. 8,8 mg per kilo body weight.
I developed a dry crusty nose and my skin dried up a little, that was about it. The symptoms was gone in a couple of days when I discontinued this dosage.
(the dry crusty nose one can find in the homeopathic proovings of borax)
However I found no benefits of taking that much; so the recommended 1/8 or 1/4 of a level teaspoon is good enough.
For those who cannot stand the taste of borax in water; fill the desired amount in a capsule, take 1 g of ascorbic acid with it, before bedtime.
This way it is easy on the stomach, not affecting the stomach acid and such.