I take a lot of b12 as I need a lot to maintain good levels. I have no side effects from them. I drink a lot of coffee and attribute occaissional palpitations to that.

For some reason I need 3500mcg of b12 in sublingual form everyday to maintain good levels. Less than that and in about 3 weeks i start with the nerve damage symptoms and difficulty sleeping, irritability, difficulty concentrating etc etc etc etc.

I am 6 foot 4 inches and a non-overweight 250lbs so i think all people are diffierent with the amount of B12 they need.

I however do experience some anxiety and worry which started when i first started to become nerve damaged and cannot tell if this is from B12 or permanent damage from when I was severely b12 deficient and sick.

I wish I could help more with an answer but the best answer I can give is that for the most part I have not had any bad experiences with my dose of B12 with regards of there being side effects or not.

My doctor swears up and down I cannot take too much and told me not to worry about it and so far i have to agree with him. I do experieince periods of euphoria that feel pretty good but sometimes i feel that heat/cold nerve damage feeling on my left side which is now normal for me after the damage done but is accentuated when i take too much iron.

i do take iron as well and that has to be monitored to avoid heavy metal poisoning as the human body has no way of to get rid of extra iron other than to store it in your tissues which causes tissue breakdown and things like heart failure.

I do have anxiety and the above heat/cold sensations on my left side when my iron dose is too high and toxicity is building up. Bloodwork thankfully tells me when that is happening although I can tell by how I feel. I call it heat/cold because it feels like heat or cold at the same time.

B12 on its own though i doubt you have anything to be concerned about honestly. I've only been on B12 since 2008 but it has been ok. I am very physically active and ride a bike, plkay hockey etc and have not exoperienced anything abnormal with regards to heart or shortness of breath or anything. I get muscle fatigue sometimes but also attribute that to nerve damage I have had from the b12 deficiency.

It absolutely would not surprise me if everyone had unique symptoms to themselves with this stuff.