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Pernicious Anaemia Society
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Low Blood Pressure and B12


I've not posted in a while. I wish it was because I had recovered but sadly I'm still battling various health issues!

I was diagnosed with low B12 last March and have been injecting regularly since May. One of my symptoms is faintness/weakness/lightheadedness as soon as I stand up and start to move around. I did a few chores today and started to feel faint and weak-I decided to check my blood pressure and it was 51/37. I sat down and about 10 minutes later it rose to 79/49. I check my blood pressure regularly-although usually whilst sitting down/inactive and it's the low side of normal 80-100/60-80. I knew that i felt very faint/weak when walking around and standing up for longer periods but I didn't know it'd be reflected quite so clearly in my blood pressure reading!

My question is - is there a link to B12 and faintness/low blood pressure? I have mild macrocytosis still - would this cause or contribute to such low/fluctuating blood pressure?

Thank you!

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Personally I wasn't aware until I just did a search that B12 could lead to low blood pressure - expected high blood pressure but not low blood pressure - though it would seem that it is a secondary effect of the anaemia caused by B12 deficiency.


This lists a lot of potential causes so it may also be that some of your other health issues are interacting.

Would you be able to mention it to your GP - the drop in levels seems significant enough to warrant some attention - though I'm far from being an expert on low blood pressure.

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Thank you for the link. I have been injecting regularly for seven months so I'm a little surprised at the continued symptoms and macrocytosis-the doctors are struggling to find a cause. I told my neurologist I'd been a vegetarian since I was five and my diet until my early twenties (when I took control of my own diet) was extremely poor-she advised I continued with B12 injections and a B complex, saying a longstanding deficiency can take a few months to recover from. I didn't think anaemia or blood pressure issues would take so long to resolve though.

I agree I think it is concerning. I didn't know if anyone else had personally experienced this-and recovered.


Hi Booksellercate. Sorry to hear that you're having such a bad time.

Just a bit about blood pressure...which can and does vary widely throughout the day.

However, at both 51/37 and 79/49 your blood pressure is far too low. To put this in context, doctors consider anything below 90/60 as a low blood pressure (or 140/90 for high blood pressure). If you were tucked up in a hospital bed as a patient, readings like those that would ensure that somebody was keeping a very close eye on you...and taking your blood pressure every 15-30 minutes πŸ˜€.

But...and this is what's impossible to know just by sharing a few words...your 'low-event' (for want of a better way of putting it) could be an extraordinary dip due to perhaps getting up suddenly, low blood sugar (had you been eating)...or a whole host of other things and as you have macrocytosis you are likely to have low blood pressure anyway...so a low on a low (if you know what I mean)...could have resulted in your really low readings.

When blood pressure is this low it puts strain on all the organs in the body since there is not enough pressure (blood pressure) to drive blood around the body properly. So...I'm not surprised that you feel so dizzy, faint, and ill.

Certain medications can cause low blood pressure (i.e beta-blockers, ACE inhibitors, diuretics, and some antidepressants - to name but a few) so,it. Ignore be worth considering if you are taking any meds that could be partly responsible.

Also (and this will be pertinent to your B12 deficiency) problems with the autonomic nervous system (the bit of the nervous system that works without our conscious control) can also cause low blood pressure - blood partly moves around the body through the action of widening and narrowing of the arteries and this is sometimes compromised when there are problems with the autonomic nervous system - potentially contributing to low blood pressure. Anaemia - any type of anaemia (including the macrocytic anaemia that you have ) is also an obvious cause as well. (Any idea what your ferritin level is like?

You mention various health issues and these could also be playing a part (obvious mischief makers are heart conditions, diabetes and Addison's disease...but there are many others.

The bottom line is that the readings you give are way too low...but the crucial thing is what your blood pressure usually runs at (given the normal variations throughout the day).

It would be a really good idea to see your GP and ask him/her to investigate and get to the bottom of your low blood pressure - especially if it is always that low.

It might help if you take your blood pressure regularly thorough tthe day for a couple of weeks (say when you get up, lunch, evening and before bed) and keep a record so that you can track, monitor, and show your GP (taking two readings - sitting and standing - is also a good idea since there should be a certain margin between the two and if there isn't this will give your GP clues about what may be going on.

Anyway...just some thoughts on blood pressure...GP visit a good idea 'cause you can't carry on feeling so dizzy and ill - especially as there is usually a solution to be had.

Take care and good luck...let us know how you get on πŸ˜€ x


Thank you for your response. I had eaten lunch and I'm on no medication which I think would have such an effect on my blood pressure.

It seems to be standing and moving around which causes these low readings. I've felt it every time for months - the lightheadedness, the 'dimming' (as I call it), the all over weakness coming on as soon as I start standing up and moving around...but I didn't link it straight away with blood pressure...because I've only tested it when I'm still, not moving around (well, you don't do you?!) so I was led into believing my blood pressure was low-normal. Today my blood pressure went from 51/37 to 110/89 within half an hour (I sat down until the faintness passed) which sounds quite wrong.

The autonomic nervous system issue sounds plausible...

I think the only thing to do, as you've suggested, is to keep a good and thorough record of my blood pressure readings - which I'll start tomorrow.


Hmm...sounds like most likely cause... anaemia....macrocytic...iron deficiency etc. Perhaps your GP would do a FBC, full iron panel, and also check your ferritin levels?

Good luck πŸ˜€

P.s. Ferritin needs to be 80 - 100 (GP's often think lower levels are okay - they're not πŸ˜€)


My doctor happily checks my ferritin...it was 85 at last reading so that's finally OK (it started at 42 but raised easily and quickly)

Thank you, I'm glad in a way to see my faintness 'registered' but wondering what this is all about!


My blood pressure is also low so I've done a few google searches about it and one thing that I keep coming up with is that low blood pressure can be caused by low potassium levels.

When I first started receiving B12 injections my potassium levels did dip just below the normal range.

I wonder if this could be an issue for you? I certainly don't know enough about it to advise you other than to suggest a sodium/potassium/chloride blood test at your GPs and to ask if this could be a possible cause of your issues.

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Thanks for replying!

I have struggled with low potassium..but I seemed to get it up - I've had my potassium checked a few times and I got it up to 4.1.

Today is the first time I've caught this very low blood pressure reading...but I suspect every episode of weakness/faintness I've had when standing/moving was possibly reflected in my blood pressure. I've had these 'episodes' consistently, always triggered by standing and walking around, since March. Would potassium give consistently low readings, or these sudden plummets? I can certainly try to increase my potassium intake with more coconut water, it certainly wouldn't do me any harm...


I'm sorry I don't know the answer to that but I think your low blood pressure does need looking at by a gp and potassium could be checked at the same time. Good luck! I'd be interested to know how you get on.

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Thank you! I'll try to remember to report back-I know it's useful to share experiences.

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