Borax is reasonably safe, but it's also reportedly toxic in sufficient quantities (if eaten), so it's not something to be careless with. You can read some introductory data here
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cause problems if used externally for short periods of time (don't soak in it...), but you'll want to be sure to keep it out of your eyes and lungs, as it can be irritating.
I'd be interested in hearing anyone's successful or unsuccessful uses of borax, too!