Hi, I should first start by saying that CFS is not a part of my diagnosis, so I hope it's okay that I'm posting here - after much searching, this seems to be one of the best places on the web for folks with b12 knowledge. I am a 33yo F. About a month ago, I went to my doctor with symptoms of recurring canker sores, fatigue, foggy thinking, trouble concentrating, forgetfulness, anxiety, trouble focusing eyes at times, and increasing gut issues. She checked only my b12 and ferritin levels. My b12 tested at 205 and MMA 0.38, likely due to 20 years of vegetarian diet, and also being lactose intolerant. I was told my ferritin was within normal range (26). My doctor suggested only 100mcg b12 daily for several weeks, and then back to down the the RDA. After reading about which types are most effective and thinking that a higher dose may be more beneficial, I bought the Jarrow methyl-b12 sublinguals - 1000mcg. Within minutes of taking one, I became so sedated that I could barely keep my eyes open, and was forced to lay down until I slowly became more alert over a couple hours. Following this, I noticed a severe decrease in mood, felt like crying, became quite shaky with some muscle pains, and felt generally 'spaced out' and weak for 24-48 hours. After feeling relatively 'normal' again, I halved the dose, and the same thing happened. After reading about the possibility of mercury amalgams reacting with methyl-b12, I decided to purchase GNC cyanocobalamin liquid B12 and started off very low at only one drop (held sublingually), which I estimate to be around 50mcg. I can take this with fewer issues (mild shakiness, irritable stomach-intermittent diarrhea, and some heartburn). At two drops, I feel a bit sleepy (though nothing like with the methyl-b12), my stomach issues intensify, and my mood drops noticeably as well. Does anyone have any suggestions as to what may be causing this? Should I keep plugging away at 50mcg until I can tolerate 100mcg with no side effects? Should liquid cyanocobalamin be effective in someone who does not have pernicious anemia, and is simply deficient due to diet? Any insights would be much appreciated. Thanks!