What is the hugest dose of B12 you can take?

Hi, I was diagnosed 3 weeks ago with B12 deficiency blood test came back 130... my doctors own words where" it's in your boots"!! and also low iron too test came back 12 for iron. I have been prescribed Cyanocobalamin 1MG tablet x 1 a day and Ferrous Sulfate 200mg 3x a day. I went to the doctors as I had been suffering with heartburn and indegestion and pain in left side of stomach and they did routine blood tests and this was the findings...I was prescribed Omerprazole for the heartburn/ Indegestion/ stomach etc which helped but still have pain in stomach so have been referred to hospital to see a gastroenterologist. Get days were I have not much energy and have definite "brain fog" and emotional for no reason just wondered if I should be on a higher dose B12? I remember at time doctor saying "I'll only prescribe X1 because of your stomache but I thought she meant iron tablets until I got my prescription and realised she obviously meant the B12... Would high doses of B12 affect your stomache?

12 Replies

  • " was prescribed Omerprazole for the heartburn/ Indegestion/ stomach"

    Omeprazole can affect the uptake of B12 in your body. See Link below.


    The symptoms of low stomach acid and high stomach acid are very similar. Sometimes people get treated for high stomach acid when they actually have low stomach acid. Which do the doctors think you have?



    Which country are you based in. Are you in the UK?

    I'm surprised you are not receiving injections. If you are UK based these documents may be useful.



    Google "BCSH Cobalamin and Folate guidelines"

    Page 29 is a flowchart giving recommmendations fpr diagnosis and treatment. I read the whole document and gave a copy to my GPs.

    Useful Websites


    01656 769 717


    Lots of useful info here




    USA website about b12 Deficiency

    useful books

    Could It Be B12 by sally Pacholok

    Very comprehensive book about b12 Deficiency

    What You Should Know About Pernicious Anaemia and Vitamin B12 Deficiency by Martyn Hooper

    Up to date book as published in 2015. martyn hooper is the chair of the PAS.

    Symptoms lists



    Symptoms Checklist in here. I ticked all my symptoms and gave a copy to my Gps.

    Blood Tests

    have you had an IFA test? A folate test? A FBC (Full Blood Count) aka CBC (Complete Blood Count?


    Other useful links



  • Thank you.... Have got both books... Ordered them when diagnosed..but haven't read fully yet and need to re read what I've already read due to not retaining information...."brain fog"??

    I live in UK...have recently joined PAS so am a member....did email them...had a reply asking for my phone number....but not heard anything back from them and that was 2 weeks ago?

    Yes have read about Omerprazole...have only had a 2 week course....never had it before....have a repeat prescription for 1x a day if need it. Yes am going to request more blood tests....am hoping that hospital referral may do more and think I will be having a gastroscopy so hopefully will see what's going on with stomach which I'm hoping may explain the deficiency.

    Thanks for reply....

  • Will sit down later and look through all those links....thank you

  • B12 (hydroxo) is used to treat cyanide poisoning - treatment is 5g (that's 5000x the normal 1mg loading dose) administered intravenously over 15 minutes with a follow up dose 30 minutes later if required.

    There is no known risk of overdosing on B12 - having levels that are two low is extremely dangerous - leading to permanent nerve damage, multiple system failure and death.

    Think most would report that B12 helps with stomach issues as low levels can manifest/be co-related with IBS. Low levels will certainly lead to problems with bowel movements.

  • Thanks for reply....so are you saying my dose not very high/ high enough? Sorry new to B12 deficiency... Learning as I go along...what do you think I should be taking?

  • There is a lot more to treating a B12 deficiency than just pumping a load of B12 into you.

    After you have had a shot your B12 levels don't really mean much unless they are still low.

    Your kidneys will remove B12 from your blood so a lot of what is put in leaves your body in your urine. There is a lot of variation in how efficiently this happens. Although on average this takes a month for cyano and two months for hydroxo this is averages. For some people it will last longer and for others it won't last anywhere near that length of time.

    It is generally put in in forms (hydroxo and cyano) that the body still needs to convert to other forms (methylated forms) and then transfer to cells where it is used and there are things that can go wrong at both of these stage, and no test that your GP will do will preduct whether something is going to go wrong at these points.

    It is most definitely the case that your GP is NOT treating you properly if your treatment is going to be driven by looking at your blood serum levels - the links sleepy bunny gave you will include standards that highlight the problems with going on test results.

    If your GP is concerned about you overdosing on B12 then that is a misconception. You cannot overdose on B12 - which was the point of the info on treating cyanide. Some studies show a correlation between B12 treatment and slightly higher rates of cancer and larger tumours but no causal link has ever been established and as a B12 deficiency can lead to cancer it is quite probable that the causal link is the period of B12 deficiency before supplementation started.

  • Interesting how different Doctors have different perspectives. My B12 has tumbled from 650 to just 110 but is described by my GP as 'slightly low'. She does, however, propose injections after my repeat bloods in a couple of weeks and won't allow me to take the high dose sub-lingual lozenges that I have already bought.

    You're on the max dose of Ferrous Sulfate so don't be surprised if you get upset stomach problems and nausea from that. Personally, I find Ferrous Fumarate easier to tolerate and top it off with a glass of orange juice to increase absorption.

    Best wishes. Clare

  • Thank you👍

  • Hi,

    Forgot to mention that an IFA (Intrinsic Factor Antibody) test can help to diagnose PA but the results are not always reliable on this test. It is possible to have a negative result and still have PA. The BCSH Cobalamin and Folate guidelines mention Antibody negative PA.

    From what I have read an oral B12 tablet may be appropriate treatment for someone whose b12 deficiency is diet related. Sometimes vegans and vegetarians can develop diet related B12 deficiency. Does the GP think your B12 deficiency is diet related? If you are eating plenty of b12 rich foods such as meat, diary, fish, shellfish,eggs then B12 deficiency is less likely to be diet related.

    If the GP suspects that your B12 deficiency is auto-immune related eg PA or an absorption issue then I am surprised that you are on an oral tablet and also surprised if you have not had an IFA test.

    Have you had a Coeliac test? A test for H pylori? Thyroid tests?

    Coeliac Disease

    People with Coeliac disease can have problems absorbing some nutrients in the gut.




    Thyroid problems and B12 deficiency can occur together. Marz knows a lot about thyroid problems if you search for her posts. The Thyroid forum on HU is very active.


    H Pylori


    I am not a medic just a patient who has struggled to geta diagnosis and wnats people to have useful info.

  • Hi... Thank you for all you info...it has been really helpful....I am compiling a checklist to revisit GP with!! Will post updates.

  • I agree with the others - surely you should be tested for PA and be on injections. As far as I'm aware the stomach symptoms could be part of the PA and the medication you've been prescribed could make it worse (but I'm not a medic).

  • Thank you

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