Doses up to 18 mg of boron daily appear to be safe for adults even if taken for prolonged periods of time. This is double the dose of even the most liberal amounts suggested by advocates of boron use. There is also no evidence to suggest that boron is either carcinogenic or mutagenic.
In 1904, human volunteers consuming greater than 500 mg of boric acid daily (this is equivalent to about 180 mg of elemental boron) showed symptoms of poor appetite and digestive problems. Symptoms of acute toxicity typically include nausea, diarrhea and abdominal cramps.