(For adults only, since it's not clear it's been studied in kids, or that there's a known safety record for taking mega-doses of niacinamide for years on end.)
Since people are taking niacin (often, a less safe form of niacin not shown to have any relevance to acne) for acne, I thought it worth pointing out you can roll your own version of a niacin treatment that was actually studied and that produced a measurable impact on acne symptoms for a significant percentage of those studied. Though sold as a prescription med as Nicomide, you can get all the ingredients off of iHerb.com (probably many other places as well):
Take one set of these per day with meal. Allow 8 weeks for results (if any) to be established.
Nicomide is one of the more successful supplements for acne/rosacea, assuming their study wasn't fudged.
Of the patients studied, 79% reported their improvement in appearance as moderately better or much better, as measured by patient global evaluation, and 55% reported moderate (26%-50% reduction in lesions) or substantial (>50% reduction in lesions) improvement after 4 weeks of treatment (P<.0001). The percentage of patients who responded to therapy continued to increase through the 8 weeks of treatment.
Of course, self reporting is a lousy way to measure an acne treatment, one designed to give your product every percentage point due to placebo effect and optimistic patient bias. Still, for people dreaming of a pill solution, it's a chance to try something that was actually studied, where there's actually some record of safety (in adults, at least).
It may be that the megadose of niacinamide acts on acne exactly as one of the downstream effects of melatonin does: by regenerating glutathione, which takes out the excess superoxide that P. acnes needs to create inflammation.