One thing to keep in mind is that taking niacin in excess of 500 mg daily may cause temporary flushing, itching, rash, gastric disturbances, lightheadedness, or unexpected drop in blood pressure.

A study found that 500 mg of NR twice daily in older adults ... The upper tolerable intake for niacin is 35 mg daily, but only to avoid flushing. NR seldom causes flushing, so you can tolerate ...