Does vitamin B3 prevent skin cancer?

Trial suggests nicotinamide has modest benefit, but only for select patients

Greg, a 75-year-old Anglo-Australian retiree, presented with a nodular basal cell carcinoma on his shoulder, which I excised.

Greg had been an avid surfer in his youth — a lot of sun exposure with no sunscreen.

Afterwards, I read in an online GP discussion forum that oral vitamin B3 (nicotinamide) might prevent further skin cancers in people like Greg. 

What is the evidence?

Clinical question

What is the effect of nicotinamide, used as preventive therapy, on the incidence of skin