Boron is a trace mineral with biological effects, though its essentiality is debated.
It seems to be able to reduce levels of inflammatory markers, and may reduce rheumatoid arthritis, osteoarthritis, and menstural cramps, though much more research is needed before we can be confident in these benefits. It also might have a small positive effect on LDL-cholesterol, but this is far from certain.
It seems to be generally safe, though human research on its adverse effects is lacking, so smaller doses may be warranted.
The lowest active dose of Boron supplementation appears to be 3mg, which is effective in supporting hormonal parameters in postmenopausal women. Studies on osteoarthritis have used 6mg of Boron while studies in youth investigating hormonal changes have used 10mg.
The optimal dose is currently not known, but the above doses appear to be active for their aforementioned goals.