Several studies have looked at the effect of niacinamide, a form of vitamin B3, on the risk of skin cancer among people with a history of NMSC or actinic keratoses (a precancerous patch of skin usually caused by excessive ultraviolet light exposure). These trials collectively suggest that niacinamide supplementation (usually around 500 mg taken twice a day) lowers the risk of NMSC (specifically BCC and SCC) in this population.[1]
One clinical trial of people at risk of skin cancer found that oral retinol (a form of vitamin A) reduced the risk of SCC, with no effect on the risk of BCC.[2] Meanwhile, several randomized controlled trials have failed to find any effect of beta-carotene (a retinol precursor) on the risk of NMSC.[3]
Finally, one randomized controlled trial of people with a history of NMSC found that selenium supplementation (200 μg per day) increased the risk of SCC skin cancer.[4] This outcome was not seen in people with lower selenium levels (≤105 ng/mL), suggesting the effect of selenium may depend on meeting a certain threshold.