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Pernicious Anaemia Society
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Can starting iron pills/B12 injections delay period?

Hi there, I started taking ferrous fumarate on the 13th November (on doctor's advice) because my serum ferritin had become very low, as it tends to do without supplementation. I started getting every other day B12 injections on the 21st November for the first time, at my GP surgery. My period was due that day, and I'm usually really very regular, but it's still not arrived, although it keeps seeming like it's going to. I'm just wondering if this could have anything to do with the new vitamins? I would have expected the opposite to be honest!

I am 44, so it's possible that it could be a perimenopause thing, but it just seems terribly coincidental timing. As always, any advice appreciated :)

11 Replies

Hi ktwing I cannot help with the period question but I wonder whether you have had your Folate level tested?

There is a complex interaction between folic acid, vitamin B12 and iron. A deficiency of one may be "masked" by excess of another so the three must always be in balance.

Symptoms of a folate deficiency can include:

symptoms related to anaemia

reduced sense of taste


numbness and tingling in the feet and hands

muscle weakness


Folic acid works closely with vitamin B12 in making red blood cells and helps iron function properly in the body.

What diagnosis have you been given - B12 Deficiency?

Do you know the cause?


Hi clivealive , yes, my folate before treatment was bang in the middle of the range, but I have started taking folic acid anyway. I'm currently on a therapeutic trial of B12, every other day for neuro symptoms and will be seeing the neurology consultant on Thursday about this. I was diagnosed with MS earlier in the year, but had/have a lot of symptoms that are suggestive of B12 deficiency - early greying of hair, iron deficiency, glossitis, breathlessness... and the haematology consultant agreed with me that although my serum B12 was within range and the IFAB test was negative, this doesn't rule out B12 deficiency, hence the trial!


Sadly the IFA test is unreliable in that it gives false negatives in people with PA half the time. So a negative result doesn't mean that you don't have PA. However, a positive result is a sure-fire, 95% certain indicator of PA.

It is not uncommon for some symptoms to appear to get worse before they get better as the B12 starts repairing the damage done to your nervous system and your brain starts getting multiple messages from part of the body it had "forgotten about" or lost contact with.

I sometimes liken it to a badly tuned radio on which you have turned the volume up high trying to catch the programme you want when all of a sudden the signal comes in loud and clear and the blast nearly deafens you.

A lot will depend on the severity and longevity of your B12 deficiency as to how long before there is no further improvement or recovery.

Some symptoms will "disappear" quite quickly whereas others may take months or even years. There is no set timescale as we are all different.

I'm not a medically trained person but I've had P.A. for over 45 years and I wish you well.


I'm not going to insult your intelligence by asking if you've thought that pregnancy might be a possibility :)

But you could ask if your surgery will do a blood test to see if you're perimenopausal? I've not heard of B12 delaying periods. But then it does so much weird and wonderful stuff, I actually wouldn't be surprised ...

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Haha, yes, I should have said - I have a coil, so it shouldn't be pregnancy! Yes, good idea - doctor was going to be my next port of call anyway :)

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This is an interesting question. Since starting injections 5-22-17, I have had a period that was after 40 days- i was concerned enough I had a pregnancy test which was negative. And i had a 36 day period. Both of these are really different for me as my period was around every 23 days over the year prior to injections.

The major change i noticed was the change in color from very dark red, verging on black, to a bright red since begining injections.

I was stunned by the new bright red color as i had just assumed the color changed/got darker as we aged. I have no idea why i thought that and never mentioned it to anyone. Im 46 and the color been getting slow progressively darker over several years. I mentiined this new bright red to my doctor. All she said was no, period blood doesnt naturally darken as we age.

Now i think it was because my red cells were progressivly getting larger and each had more hemeglobin making it darker but that could be a completely wrong assumption.

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Thank you, that's actually quite reassuring - I've not been able to find anything online except people's periods getting heavier on B12, which is what I was expecting. Of course maybe when it comes it will be heavier!

Interesting about the colour, when I took a couple of weeks of sublingual methylcobalamin a while ago, my period then was very red too, maybe it's the same reason.

Wild how complicated these bodies are!

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My periods have changed since starting B12 injections. I was always, exactly, on a 28 day cycle. Now I'm more like every 5 and half weeks, which is very annoying, the not knowing exactly when it comes! I'm 43years so could be heading towards the menopause too.

Also bear in mind having low iron can actually CAUSE heavy periods too! So get it checked regularly.

Weird this B12 deficiency, isn't it? Lol!

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Thank you, again that's reassuring to know!

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pretty sure the frequency of my periods didn't change and they actually got lighter rather than heavier.

B12 deficiency can cause a false menopause

It may be co-incidence. It could also be related to the iron supplementation, or it could just be the way your body has responded to B12.


I started B12 injections in may 2018 it’s now a 1/3 through sept and I’ve had one normal period back in may and two, 2 day periods in June and July nothing in August and nothing as of yet although can safely say I’m not pregnant ( tested yesterday) but I’m having symptoms of a period ..... mood swings ,sugar craves for about 5 days a month when I should be due on ..... I’m 37 and due my B12 in two weeks so am gonna bring this to my surgery’s attention


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