In smokers, high doses of vitamin A supplementation can accelerate cancer due to carotenoids being cleaved into DNA-damaging products in the oxidative lung environment.

“it's been shown that the lungs of smokers are a highly oxidative environment and in combination with deficiencies in certain antioxidants such as vitamin c and e it has been shown that carotenoids which are part of the vitamin A family under these highly oxidative conditions can actually get cleaved into products that damage DNA which has been shown to cause cancer and in fact these cleaved products also inhibit the ability of these carotenoids to be formed into the vitamin A hormone”