ONTRAC: oral nicotinamide for skin-cancer prevention (RCT, 2015)

386 people who had already had at least two non-melanoma skin cancers took either nicotinamide 500 mg twice daily or an identical placebo for a year, with neither they nor their doctors knowing which. New skin cancers were 23% less common in the nicotinamide group. Two details matter for using this: the benefit disappeared once people stopped taking it, and everyone in the trial was already high-risk, so it does not tell you what nicotinamide does for average skin.