The upper intake level for niacin (vitamin B3) is 35mg/day, with the recommended daily intake of 20mg/day. You have not mentioned any past medical history, therefore I presume you are taking this as a supplement. If this is the case, 500mg is excessive, in addition to that in your normal diet. Niacin can be taken in higher doses in certain circumstances for example treatment of high blood lipid levels. Doses > 2.5g per day over time can cause liver damage and glucose intolerance. It is recommended therapy should be discontinued if liver function tests (transaminases AST and ALT) rise >3 times the upper limit of normal. I would recommend discussing the need for continuing therapy with your family doctor.
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