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Pernicious Anaemia Society
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New Member - Query re. B12 Injections Stopping B12 Getting To Cells Unless Serum B12 High

Hello, I'd be grateful if someone could help me with this query. I read in a recent post that B12 shots could cause problems with B12 getting into cells unless serum levels were very high. Is this due to sensitisation of the cells because of the huge level of B12 in the bloodstream for some time following the injections, or is it because the digestive system has been bypassed, or some other reason entirely?

I have recently started using a methylcobalamin mouth spray and this information worried me - could it cause issues with B12 absorption from food in the future? I have no reason to believe that I have PA, but - I'm a vegetarian; I take omeprazole daily due to Barrett's Oesophagus (and my B12, folate and ferritin levels were very low even before starting it); I had a sub-total thyroidectomy in 1985 due to Grave's Disease and my medication is not optimal; I had a gastroscopy using nitreous oxide in 2016 and three days later experienced burning tongue and mouth plus epigastric symptoms which went on for seven months, and I haven't been really well since.

Thanks in advance for any responses.

4 Replies

not clear why it happens but seems to be genetic - and the trigger is the high serum B12 - and the most common cause for that is injections but it also means that symptoms of B12 deficiency with high serum B12 is recognised as a warning sign of some conditions that can cause serum B12 to build up.

it doesn't affect your absorption but if you are one of the people who has this reaction then it will affect your ability to use B12 once it has got into your blood stream.

Ideally you shouldn't supplement with high doses unless you know you have an absorption problem - though it is highly likely from what you describe that you do have an absorption problem. tablets and sublinguals can be an effective way of maintaining levels once they have been restored but is likely to be very slow at rebuilding levels.


Gambit, thanks for your response, it's much appreciated. I've only been taking the oral spray for two weeks so would it now be too late to get a B12 test do you think? I have also been taking a B complex tablet and Floradix for about three weeks.

Following an extraordinarily long coughing infection last winter the burning mouth etc. had returned and, after suffering for over six weeks and spending a small fortune in the health shop on all sorts of things, the only thing that had any effect at all was the methylcobalamin spray. My mouth was a lot better within two days and virtually back to normal after a week. My stomach problems are easing too. I had put it all down to silent reflux and only thought of B12 after following a link regarding nitreous oxide on the ThyroidUK forum. I think the nitreous oxide tipped me over the edge and I've not been the same since.

As well as being vegetarian I do have a poor diet as I don't have much appetite. I am hoping that by addressing this and supplementing I can get my B12 levels up.

Thanks again for your help.


it is going to be very difficult to interpret blood tests if you have been using the spray - levels are likely to be really high.

The silent reflux could be a symptom of low stomach acidity which of itself would affect B12 absorption - but is also a potential consequence of PA, the most common B12 absorption problem.

It might be worth asking if absorption problems could be looked into - IFA is a possible test as long as you stop supplementing for a couple of weeks but it's extremely insensitive and gives false negatives 40-60% of the time so a negative is a long way from showing that you don't have PA.

Other absorption problems would include SIBO, h pylori, coeliacs and crohn's.


Thanks again Gambit. I visited my doctor a few weeks ago as I thought low thyroid levels might be the culprit. She also did a full blood count, so would the RDW give an indication of B12 levels at all? The test was taken before I took any supplements.


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