Nicotinamide (vitamin B3) has been shown to have protective effects against damage caused by UV radiation and to reduce the rate of new premalignant actinic keratoses.METHODS: In this phase 3, double-blind, randomized, controlled trial, we randomly assigned, in a 1:1 ratio, 386 participants who had had at least two nonmelanoma skin cancers in the previous 5 years to receive 500 mg of nicotinamide twice daily or placebo for 12 months.