IThere was no evidence of toxicity in people given 10mg of boron a day for long periods, concluded this study. In healthy people, boron is rapidly absorbed and then excreted in the urine.

Another study [1] put eleven postmenopausal women onto a diet delivering 0.3mg of boron a day, then gave them a daily 3.0gm boron supplement. Although the researchers had effectively increased the women's boron intakes tenfold, their blood plasma boron levels only increased by half. This suggested that the body was good at taking what boron it needed then excreting the rest.

Contraindication
In individuals with kidney problems, boron intake, as with any mineral supplement (particularly potassium) should be carefully monitored as impaired kidney function could reduce excretion, resulting in boron accumulation.

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Ed.- The Tolerable Upper Intake Level for boron set by the US Food and Nutrition Board of the Institute of Medicine for adults over the age of 18 years has been established at 20mg/day .[2]