Thyroid UK
87,636 members102,331 posts

Blood test for B12. I am taking a supplement - when is best time for blood test?

I have been taking 1000ug of B12 for just over 2 months. It is the least effective sort (ie not the methyl one) and in tablet form. It is described as slow release. I normally take it in the evening before I go to bed. When is the best time to take a blood test? I am keen to see how I am doing as previously my B12 was at the bottom of the normal range.

3 Replies

Hi. I seem to remember a discussion about this some time ago, and the message that I took away was that you really need to stop the B12 about 5 weeks before a test to get a true picture. Of course, I could have remembered this incorrectly, and am unable to find where I got this number from! Looking online, wikipedia says the elimination half life is 6 days, so I guess you should probably stop at least a week before the test to get a meaningful result, and longer to get a absolutely accurate result. Of course, the B12 test is not truly accurate anyway - but that's another story...!!!

I understand that all forms of B12 will show up the same in the blood test.


I wonder if it might be ok not to bother with a test unless you really want one, as the supplement is cheap and the excess will be excreted if your body does not need it. I use patches bought on amazon and find them good, as you absorb better through the skin than the stomach if you have problems with intrinsic factor.


If you are unable to absorb B12 the tablets will pass through your system and not make a difference which is why most people who are B12 deficient have jabs or use a B12 spray. These sites have a wealth of info that you might find helpful.



You may also like...