Hi there, I've been taking B12 5000mcg along with B complex and multivitamins plus Zinc since last August (9 months) and of recent I haven't been feeling too well. Slurred speech, feeling under the weather, like I'm operating in slow-motion, fuzzy headedness. I've also been on pain killers for back pain so I stopped all the medication for that and the B12 simultaneously to see what's what. When I re-introduced the B12 (but not the pain killers) after 48 hours, I had a weird surge after sucking on the pill for a few minutes like alcohol, like my body was getting something it "needed" and then a few minutes later a sick feeling like over-dose. Today I was going to take a half tablet. Has anyone else experienced a weird change in B12 requirements after such a long period? Does the feeling of taking a tablet after missing a day indicate some kind of habituation bordering on a psychoactive effect? Any thoughts appreciated. When I first started taking B12 my energy levels were sky high at the time, but it's difficult to say, with complex diet changes ,IBS etc, what's going on in such a complex set of symptoms and diet changes and never seeming to nail anything down.