If you have low B12, you may have a gene mutation that impedes absorption, the MTHFR gene, there is a simple test for this…ask you doctor. There is an active form of B12 that is more easily absorbed.
Also, On a Facebook group for itch with no rash…some discovered that when they took a preworkout supplement they did not itch…Beta Alanine is a otc supplement that kills the itch for me for 5+ years…I buy the NOW brand on Amazon and take 750 mg every four hours as needed. The preworkout dose is higher and can cause skin tingling so start slow. Two Stanford pediatric dermatologists wrote a paper on beta Alanine and itch.