B12 is a huge molecule. Whether it passes successfully thru the skin, is iffy.

There have been spammers who have come here pushing the patch. Their "studies" involved 12 people! I found that
insufficient.

Since oral of B12 has been shown to raise blood levels in studies
done with humans, why buy a more expensive thing to do the same thing. Oral methylB12 (active form) is pennies a day and very low cost.

The patch in US contains selenium in 200mcg quantity and cyano form of B12. Not everyone needs that selenium (depending on diet) and cyano form is synthetic and requires activation to methyl form to work. Some people cannot convert it or are sensitive to the cyanide portion of it.

If you try this patch, get tested before you do, to see your base line level, and then test in 3 months. Do not take any oral vitamins or fortified cereals. (they have B12 in them commonly.)
Patches are an expensive way to go, if they work at all.