Vitamin B6 is usually safe, at intakes up to 200 mg per day in adults.⁴ However, neurological side effects can sometimes occur at that level.⁵ Levels higher than 200 mg are more likely to cause such problems. Vitamin B6 toxicity can damage sensory nerves, leading to numbness in the hands and feet as well as difficulty walking. The National Academy of Sciences performed an analysis of vitamin B6 studies. They determined the safe upper limit for long-term use is 100 mg per day. However, under supervision of a healthcare professional, up to 200 mg per day of vitamin B6 can be safely taken by most men and nonpregnant women for limited periods of time. Pregnant and breast-feeding women should not take more than 100 mg of vitamin B6 per day without a doctor’s supervision.

In a double-blind trial, people with diabetes who also had with kidney disease received a daily placebo or 2.5 mg of folic acid, 1 mg of vitamin B12, and 25 mg of vitamin B6 for three years. Compared with the placebo, vitamin supplementation accelerated the decline in kidney function and increased the incidence of cardiovascular events (such as heart attacks) and heart disease-related deaths.⁶ Based on this study, diabetics with kidney disease should not take these vitamins without a doctor's supervision.