No, never tried it. I'm getting good results w/o it, so don't want to multiply supplements. If, for some reason, I need to drop niacin in the future, I'll give it a go.
I forgot to mention that I do take fish oil capsules -- six a day, which is probably more than I actually need. But, they are plentiful and cheap ...
And, on the subject of fish oil capsules, I stumbled onto a dietary "solution" to my Cavalier Spaniel's furious itching/scratching (not related to fleas, btw) -- a dietary supplement for dogs [eyes rolling back in head]. Reading the fine print at the product's website indicated that omega 3 and 6 fatty acids were a big component of this supplement.
So, since I already have a lot of that on hand anyway, I've been dosing the dog with two a day -- one in the morning, another in the evening. It seems to be working! We're only a week into the experiment, but she's scratching way, way less than before.