Answers:
You may own a mild allergic aversion to something but it might not be the vitamins, it could be something in the formulation or something else entirely. If you are taking megadoses adjectives bets are stale though. Changed your brand of shampoo? Conditioner? Laundry powder? Soap? Skin cream? Clothes? Bought a strange keep watch on? Ear rings? Some ancestors are sensitive to nickel which is contained by some of the alloys used within watches and guardian ear rings.
Ordinary multivitamins in low doses probably safe. Maybe adapt the brand you use and see what happen.
It's any the vitmins or the crystal meth.
It's possible that you enjoy an allergy to one of the ingredients. Stop taking the vitamin and see if the itching go away.
Perhaps. Read the ingredients sticky label: Even if the vitamins don't bother you, within might be something in the remnant used to form the supplement.