Nutshell answer:
Niacin is a water-soluble B vitamin and the common name for two very different compounds: nicotinic acid and niacinamide(you don't really need to know that). Like all B-vitamins, niacin plays a role in many aspects of energy metabolism and nervous system function. Furthermore, Niacin has been shown to provide better circulation and cardiovascular health.
There are many drinks on the market that have Niacin in them. The draw back is that due to increased dosages (in one drink in particular) a niacin intolerance can occur leaving the skin flush or hot feeling as well as having potential liver damage(not a good thing). With that being said I am not recommending you stop use of ED's that contrain Niacin. Just be careful of over dosing. Start off with no more than 250mgs per day for a week and then gradually increasing the levels you take in to about 2000mgs. This will give the body time to acclimate to the niacin and it will be able to better metabolize the higher doses.
Whether you think can or think you can\'t, you\'re probably right!