Niacin causes flushing in some people. Taking an aspirin a 1/2 hr before the niacin will help with that. It can block the reaction in most people. 100mg is not a high dose, as far as toxicity goes with niacin.

The 500mg doses of Niaspan are sustained release. For what it is worth, the Merck Corp is making a new sustained release Niacin for cholesterol lowering,
and for raising HDLs which it does better than statins. They are working on a patent with a second drug added to block the flush.

The KOS corporation is also working and an additive to block flushing.
Here are some flushing tips from their website:
http://www.niaspan.com/Niaspan_Flush...shing_Tips.asp

Also, the future is going to be Niacin and other HDL raising drugs. The most current thought (now that most of the statins are generic) is that lowering LDLs does not protect as well as raising HDLs.. Just think of all those people on statins...with all that toxicity. So the new drugs in the "pipeline" are all HDL raisers. You can do this effectively yourself with fish oil.

Long term use of high dose niacin should be monitored by a doctor, and not self administered, since some dosage forms (sustained release) show negative effects on the liver.