If you take any form of niacin in high doses you do need to get proper amounts of vitamin b6 too. Without enough vitamin b6 you can have some problems with brain chemistry.

Niacinamide-adenine dinucleotide (NAD) is an active enzyme that is required for the proper function of vital areas of the brain. In schizophrenia, there appears to be a failure to deliver enough NAD to the brain. Vitamin b3 is required for the transformation of tryptophan, an amino acid, into NAD. If there is a niacin deficiency, this necessary transformation of trypotphan into NAD is inhibited, and there is not only a NAD deficiency established, but there is also an overload of tryptophan in the brain’s chemistry. Tryptophan is considered to be one of the most toxic of amino acids. An overload of it in the brain can be very harmful, especially if it is not properly converted into NAD, because it can cause undesirable perceptual and mood changes. If there is a b3 deficiency, for whatever reason, the consequent NAD deficiency will lead to ever-increasing tryptophan overload uninterruptedly unless and until the proper levels of b3 are given.

Pyridoxine, or vitamin b6, is used in the treatment of cerebral allergies by many Orthomolecular physicians. There is clinical evidence that pyridoxine is involved in the tryptophan-niacin metabolism previously explained. Morever, b6 is a precursor to over 60 enzyme reactions, is necessary for the proper metabolism of all amino acids, and is required for the maintenance of a stable immunologic system.

Source: Brain Allergies: The Psychonutrient Connection by William H. Philpott, MD and Dwight K. Kalita, Ph.D.

Most cases of high doses of niacin I know about are recommending the nicotinic acid form.

I used to take 5 grams of nicotinic acid per day with other nutrients like b6 and the other b vitamins. My histamine levels are balanced now. I now take about 1,500 mg. of niacinamide and 200 mg. of nicotinic acid per day.

The flusihing is actually your mast and basophil cells filling up with histamine. I found references to low histamine levels (or histamine deficiency) associated with MS. There is a simple test to find out if your histamine levels are low or high. Just buy 50 mg. of nicotinic acid. If that dose (taken on an empty stomach) causes a flush then you have high histamine levels. But if it doesn't then you probably have low levels. (You can try testing again with a higher dose to make sure that some dose will cause a flush for you.) People with high histamine levels should never use nicotinic acid.

Now that my histamine levels are balanced I don't tolerate the flush as well as I used to. But I took (5 grams of) nicotinic acid for almost two years getting a flush every single morning with that first dose and didn't mind the flush at all. I honestly think that the flushing form is better for some reason; maybe something to do with the liver. Although liver enzymes will be slightly effected by high doses of nicotinic acid.

I will use amylase, a digestive enzyme for carbs, to counter the effect of the flush. Amylase lowers histamine levels.