Thyroid UK
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Does anyone know if it is best to stop supplements before b12 and iron blood tests?

I am going for iron and b12 blood tests in about 6 weeks would it be better to stop supplements before? If so how long before? I would prefer injections if poss especially iron because its not easy to take and gp said b12 pills don't work anyway but I take methylcolbamin which are expensive so injections would be good for b12 too!

4 Replies

For B12 testing info, this is a good document :

This paragraph discusses how long you should be off B12 before testing :

"Patient preparation

Patients should refrain from taking nutritional supplements for 24 hours before venipuncture. If supplements containing vitamin B 12 have been taken in the month prior to venipuncture, details should be given on the request form. Recent i.m. B 12 therapy may also complicate interpretation of the results. "

For iron I think 7 days off is necessary - see :


Thanks xx


I understood it was necessary to stop B12 4-8 weeks prior to testing.

Don't know about iron.


That probably makes things easier, but there may be people who simply can't manage to go that long without supplements. So, informing the lab about your supplementation, as is suggested in that paragraph I quoted, seems to be a good compromise.


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