If your doctor says you should be taking it, I would start with 1000mcg. Make sure you get methylcobalamin in a sublingual. I use Jarrow, and have used Country Life, which is very good too. I started at 1000mcg, then went to 3000mcg, and now I take 5000mcg. I have neuropathy, and found the B12 has helped significantly slowed the progression of the neuropathy. While taking the 3000mcg, my B12 level dropped 200 pts, so I switched to the 5000mcg. The best B12's, usually can not be found in the store. I shop at www.iherb.com