I'm posting a link to an article about this.
But it's not just niacin like would come out of a tablet/capsule, there's a special form the study used (don't open up capsules and put on skin - real niacin will cause intense reddening and "flushing").
This study was the "gold standard" double-blind, placebo controlled type (people using the active ingredient didn't know it and neither did the researchers). They studied 0.5% octyl nicotinate, and 5.0% myristyl nicotinate, suspended in silicone. They put 6 drops on the scalp each night. Results were documented by photography. The researchers were not certain whether the improvement was a result of improved hair quality, or actual hair regrowth (they needed a different measuring method, but in women, it's not practical to shave a little section to count how many hairs evolve and grow or are just resting), and they were also not certain if one or the other of the forms of nicotinate in the treatment was more effective, or it the combination was necessary, or if a different formulation would be more effective.
Myristyl nicotinate is available in a cream (by prescription). It has been used for skin cancer.
Frederic Fekkai MORE Scalp Purifying Shampoo and conditioner has some of these niacin forms, but the percentage is unknown, so their claims aren't backed up by real data.
Definitely look at the picture in the article.
Taking niacin orally isn't going to do a lot - if you take the regular "niacin" alone in big doses, you'll feel flushed and burning all over because it dilates blood vessels. It's important to get enough, but megadoses aren't a good idea. Niacin is prescribed for high cholesterol, but that's a prescription and is monitored by a doctor because it can be very hard on the liver.
Sorry that's so long. I love research and I have fine, thin hair and the women in my family all get thinning hair so it's of great interest to me!
I know there are niacin creams for the face which do seem to help redness and "age spots" to a small degree, but I do not think it's the same form of niacin - they call it "niacinamide" and that is a different form of the vitamin.