B12 can cause dry eyes when you're lacking the B12 vitamin.
Having a B12 deficiency is associated with severe dry eye and eye pain.
B12 does help with eyesight and can help prevent macular degeneration which is a condition in which your retina deteriorates over time, causing blurry vision.
When you see cobwebs in your eyes it means that the Vitreous "which is a jelly like material in your eyes" becomes more liquid.
And when this happens the microscopic collagen fibers within the vitreous tend to clump together.
These bits of debris cast tiny shadows onto your retina, and you perceive these shadows as eye floaters.
Strings, blobs, and cobwebs: they all describe types of specks that may appear to float across your field of vision.
These "floaters" are usually just a nuisance.
"In many cases, floaters become less noticeable or more tolerable over time, and can even disappear entirely.
Although cobwebs are a common floater type, you may see more of them after a detachment.
It may also appear as if you are viewing the world through a mist.
Keep in mind that a sudden increase in floaters, a decrease in vision, and flashing lights can be signs of a retinal detachment.
The floaters often subside starting within a few days, and all but a few settle to the bottom of the eye and disappear within a 6-month period. Some residual floaters can be seen for life.
If the floaters are a major nuisance or severely hinder your vision, the best way to get rid of them is through either vitrectomy or the use of lasers.
A vitrectomy is a procedure in which your doctor will remove the gel-like substance (vitreous) that keeps the shape of your eye round.
They mainly don't negatively impact vision and are more of an occasional annoyance.
In time, you may even learn to ignore them even if they don't disappear entirely.
As a result, treatment is rarely recommended for common floaters if they aren't impacting vision significantly.