Never tried this myself but here's from an article I found on a quick search-

How to Take Borax Internally

Earth Clinic's Ted recommends that men take 1/4 teaspoon (940 ~ 1,000 mg) of borax dissolved in one liter of water. This liter of water is then sipped throughout the day.

Ted recommends 1/8 teaspoon (470 mg ~ 500 mg) of borax dissolved in a liter of water, sipped throughout the day, for women. This borax water is taken 5 days on and 2 days off. For example, take the Borax water Monday through Friday and take the weekend off.

Some people add the borax to their morning coffee or sprinkle it on their food as they would salt. It would be wise to measure out your daily dose (1/4 teaspoon for men (940 ~ 1,000 mg), 1/8 teaspoon (470 mg ~ 500 mg) for women) each morning if you choose this method to make sure you are not taking too much.

While borax is no more toxic to the body than table salt, taking too much may cause temporary stomach upset or diarrhea. Even the regular dose may be too much for some. If you find that the ¼ or 1/8 teaspoon daily does not agree with you, cut the dose in half. Or you can use the much more diluted version recommended by Walter Last, which many have found to work quite well for their needs.

Walter Last recommends dissolving 1 teaspoon of borax in 1 liter of water and then taking one or 2 teaspoons of the borax solution daily.

https://www.earthclinic.com/remedies/borax.html