Retin-A on breasts safe?

I have read about the benefits of Tretinoin including increased collagen, firmness, and increased elasticity to the skin. My question is, would Retin-A applied to the breasts have the same effect? Would it be safe to do so? ( I use it on my face nightly and love the results) My breasts are drooping slightly and I want to wait as long as possible for a lift. Even if it won't correct sag will it prevent future sagging?