I've been taking borax for about 2 years, for a fungal infection that adapts to almost everything.
It has never adapted to borax, but the borax does little more than reduce the symptoms as long it's taken; when it's not taken the symptoms return in a matter of days.
I started on 1/4 tsp of laundry-section borax in a liter of water consumed throughout the day. Ramped it up to 1/2 tsp in the same amount of water. Probably equates to about 2.5 grams per day for 1/2 tsp.
On 1/2 tsp, after a few weeks I broke out in red welts on my back, which disappeared a few days after stopping the borax intake. Not really a problem, but clearly an undesirable reaction.
More concerning was a low-grade kidney pain on the right side with long term consumption. Occurred after a year or so and now reoccurs after taking it for about a month.
Because of this, I now take it only occasionally. It still works, but no longer see any reason to expect it to eradicate the fungal infection.