Doses of niacin as low as 500mg may cause liver inflamation, but this effect is more common when the dosage reaches 3g.
For diabetics, Glucose control may become more difficult.
Niacin is a vasodilator -- watch for a lowered BP.
Large doses of pure niacin sometimes result in a headache, and flushed complexion.
Large doses of riboflavin can cause the urine to become very yellow.
While vit B complex is generally harmless or helpful, this is Not a med to experiment with at megadosages. Stick to the RDA.