Sunscreen won't cause B12 deficiency.

No.

Vitamin B12 comes from food—meat, dairy, fortified stuff—not the sun. Your skin can’t make it. Sunscreen blocks UVB, which your body uses to produce vitamin D, but that’s a different vitamin entirely. So if you’re worried about B12, look at your diet, not your sunscreen.

As for “other health issues”: the chemical filters in some sunscreens (oxybenzone, octinoxate) get absorbed into the bloodstream. That sounds scary, but the FDA and dermatology consensus is that the cancer-prevention benefit far outweighs any theoretical risk. If you’re still uneasy, use a mineral sunscreen with zinc oxide or titanium dioxide—they sit on top of the skin and don’t absorb.

Skin cancer is a much bigger risk than sunscreen chemicals.