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What not to do after a B12 shot?

Vitamin B12 (injection)

Medically reviewed by on Nov 28, 2022. Written by Cerner Multum.

What is Vitamin B12 ?

Vitamin B12 is used to treat vitamin B12 deficiency in people with pernicious anemia and other conditions.

Vitamin B12 may also be used for purposes not listed in this medication guide.


You should not use Vitamin B12 if you are allergic to cobalt, or if you have Leber’s disease.

Before taking this medicine

You should not use this medicine if you are allergic to Vitamin B12 or cobalt, or if you have Leber’s disease (an inherited form of vision loss). This medicine can lead to optic nerve damage (and possibly blindness) in people with Leber’s disease.

Tell your doctor if you have ever had:

  • eye problems or Leber’s disease (in you or a family member);
  • kidney or liver disease;
  • iron or folic acid deficiency;
  • any type of infection; or
  • if you are receiving any medication or treatment that affects bone marrow.

Tell your doctor if you are pregnant or breastfeeding. Your dose needs may be different during pregnancy or while you are nursing.

How is Vitamin B12 given?

Follow all directions on your prescription label and read all medication guides or instruction sheets. Use the medicine exactly as directed.

Vitamin B12 is injected into a muscle or under the skin. A healthcare provider may teach you how to properly use the medication by yourself.

Your dose needs may change if you become pregnant, if you breastfeed, or if you eat a vegetarian diet. Tell your doctor about any changes in your diet or medical condition.

Always follow directions on the medicine label about giving Vitamin B12 to a child. Your child’s dose will depend on age, weight, diet, and other factors.

For pernicious anemia, you may have to use Vitamin B12 for the rest of your life. Do not stop using the medicine unless your doctor tells you to. Untreated vitamin B12 deficiency can lead to anemia or irreversible nerve damage.

Pernicious anemia is also treated with folic acid to help maintain red blood cells. Folic acid alone will not treat Vitamin B12 deficiency or prevent possible damage to the spinal cord. Use all medications as directed.

You will need frequent medical tests.

Vitamin B12 can affect the results of certain medical tests. Tell any doctor who treats you that you are using Vitamin B12.

Store this medication at room temperature away from moisture, heat, and light.

Use a needle and syringe only once and then place them in a puncture-proof «sharps» container. Follow state or local laws about how to dispose of this container. Keep it out of the reach of children and pets.

What happens if I miss a dose?

Call your doctor for instructions if you miss a dose.

Get your prescription refilled before you run out of medicine completely.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

What should I avoid while using Vitamin B12?

Avoid drinking large amounts of alcohol while you are being treated with Vitamin B12.

Vitamin B12 side effects

Get emergency medical help if you have signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.

Vitamin B12 may cause serious side effects. Call your doctor at once if you have:

  • heart problems—swelling, rapid weight gain, feeling short of breath;
  • fluid build-up in or around the lungs—pain when you breathe, feeling short of breath while lying down, wheezing, gasping for breath, cough with foamy mucus, cold and clammy skin, anxiety, rapid heartbeats; or
  • low potassium level—leg cramps, constipation, irregular heartbeats, fluttering in your chest, increased thirst or urination, numbness or tingling, muscle weakness or limp feeling.

Common side effects of Vitamin B12 may include:

  • diarrhea; or
  • swelling anywhere in your body.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

What other drugs will affect Vitamin B12?

Other drugs may affect Vitamin B12, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell your doctor about all your current medicines and any medicine you start or stop using.

More about Vitamin B12 (cyanocobalamin)

  • Check interactions
  • Compare alternatives
  • Reviews (22)
  • Imprints, shape & color data
  • Side effects
  • Dosage information
  • During pregnancy
  • Support group
  • Drug class: vitamins

Patient resources

  • Drug Information

Common questions about hydroxocobalamin — Brand names: Cobalin-H, Neo-Cytamen

Hydroxocobalamin is a manufactured version of vitamin B12.

It works by increasing your levels of vitamin B12 so that you can make red blood cells that work properly to carry oxygen around your body.

This helps to reduce symptoms such as tiredness and lack of energy.

When will I feel better?

Hydroxocobalamin starts to work straight away. However, it may take a few days or weeks before your vitamin B12 levels and symptoms (such as extreme tiredness or lack of energy) start to improve.

Are there any long-term side effects?

It’s OK to have hydroxocobalamin injections for a long time. Some people will need to have them for the rest of their lives.

What will happen if I stop having the injections?

You’ll be given appointments for your hydroxocobalamin injections.

If you miss having your injection, the level of vitamin B12 in your body will go down even more. This may make your health problems worse.

Talk to your doctor if you want to stop having hydroxocobalamin injections or if you are bothered by side effects.

Will it affect my contraception?

Hydroxocobalamin will not stop any type of contraception working, including the combined pill and emergency contraception.

Contraceptive pills may reduce the levels of hydroxocobalamin in your blood, but this will not affect your treatment.

However, if hydroxocobalamin makes you sick (vomit) or have severe diarrhoea for more than 24 hours, your contraceptive pills may not protect you from pregnancy.

Can I drive or ride a bike?

Hydroxocobalamin generally does not affect your ability to drive or ride a bike.

However, it can make some people feel dizzy. If you feel dizzy after having it, do not drive, cycle or use any machinery or tools until you feel OK again.

It’s an offence to drive a car if your ability to drive safely is affected. It’s your responsibility to decide if it’s safe to drive. If you’re in any doubt, do not drive.

Talk to your doctor if you’re unsure whether it’s safe for you to drive while having hydroxocobalamin. GOV.UK has more information on the law on drugs and driving.

Can I drink alcohol while having hydroxocobalamin?

Yes, you can drink alcohol while having hydroxocobalamin. Alcohol does not affect how this medicine works.

However, drinking too much on a regular basis can cause folate deficiency anaemia.

Try not to drink more than 14 units of alcohol a week. A standard glass of wine (175ml) is 2 units. A pint of lager or beer is usually 2 to 3 units of alcohol.

Is there any food or drink I need to avoid?

You can eat and drink normally while having hydroxocobalamin.

More in Hydroxocobalamin

Page last reviewed: 13 October 2022
Next review due: 13 October 2025

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I feel terrible AFTER my injection

Some people complain that, at just the time when you think you should be feeling better, you feel terrible. It’s usually during the loading doses (the first few days or weeks of starting treatment, after diagnosis).

How often does this happen?

We think about 1 in 10 people get this effect. So you will hear about it if you read group chat like Facebook New Beginnings, but it’s not very common.

Most people affected suffer from skin rashes. Some do get a neurological effect (pins and needles, burning pain, muscle titches), and some get heart palpitations.

Who gets these symptoms? We couldn’t use anything to predict who would get what, in advance; it’s just one in 10 people on treatment.

What sort of Symptoms could I get, and what should I do about them?

We think it happens because the B12 allows the body to put itself right, and if you have had things wrong for a long time, then there’s a lot to put right in a small period of time.

Skin rashes

This may take the form of a reddish colour like psoriasis, lumps (a bit like a large version of nettle rash), spots (acne). The skin is actually the largest organ in the body, and when the body wants to excrete a lot of a substance quickly, it often sweats it out. In this case, we think the culprit is homocysteine, which is also known as the Depression chemical, so you may feel unaccountably happy about your dreadful appearance.

But why does the body have a depression chemical? It’s possible that it’s original purpose was to keep us hidden away in caves during the long northern winters, so we didn’t get eaten — there was no point in going out anyway because there was nothing to eat, a sort-of hibernation chemical. But homocysteine is also an important chemical on the pathway for creating some of the amino acids, so it’s being produced all the time, and when there’s enough B12 around, then it gets converted into the next stage, which is the happiness chemical SAMe (S-Adenosyl Methionine).

So having enough B12 not only removes the depression chemical, but actually converts it to the happiness chemical. Just for info, homocysteine build-up is associated with heart disease and petrified arteries, so it’s as well to get it out of your system. And yes, one of the tests for B12 when we thought it could be diagnosed entirely from blood tests is to test for elevated homocysteine.

What do I do about it? Some people put E45 or similar cream onto their skin to reduce the itching feeling. I personally don’t recommend this — you don’t want to trap the homocysteine next to your skin you want it removed.

I think it’s better to wipe your skin over with water to remove the homocysteine, eg a flannel bath (a few times a day) ro a shower (which you won’t want so often).

If you are having loading doses, then this effect shouldn’t last more than 3-5 days.

Burning sensation, pins and needles

if you’ve done a lot of reading on B12 deficiency, you’ll know that the myelin sheath of the nerve axion breaks doen when there isn’t enough B12, and when the myelin sheath isn’t htere, then the nerves stop conducting. This accounts fo the sensation of numbness, or loss of power (paralysis) that many people experience as B12 deficiency progresses. Good news is, it is reversible!

So, you’ve got a whole lot of parts of the body where the brain doesn’t get many signals, eg skin of lower arm or shin/ calf, hands, feet may all have limited feeling — since arms and legs tend to not worry about feeling things you might not have paid much attention to the loss of feeling, and hands and feet have lots of nerves so the loss of a few might not have come to your attention.

Suddenly, these nerves start repairing themselves, and the brain starts to get signals from nerves that it thought had already died. What’s a brain to do? The default response from the brain is to say «pain» until it’s learnt what it actually is. So you rbrain interprets a whole lot of pain coming from your limbs, even though when you touch what you think is the affected area, it doesn’t seem to do much.

Over a few days, your brain will start to make the connection. «That signal from my right arm, that comes when I jiggle my fingers. It doesn’t cause any damage, so i’ll change my interpretation from pain to jiggling fingers».

This may last a little longer than a few days, because it’s unlikely that all of your nerves will repair at exactly the same moment. So some will repair, you’ll go through that bit of confusion (ie thinking it’s pain when it isn’t), then the next nerves will repair, and you’ll go through the same process.

What can I do about it? Not a lot. Apart from to keep taking the supplements (see below).

Heart Palpitations, feeling nauseous

B12 deficiency is often acompanied by a whole lot of different deficiencies. Some of the most common are Vitamin D deficiency (in part because many sufferers don’t go out into the sun so they don’t get it replenished naturally, but also an absorption effect), hormone deficiencies (most commonly hypoadrenalism leading to low cortisol, ie you feel tired), an dmineral imbalances such as too little potassium (K) or too little magnesium (Mg) (because it takes work to maintain more potassium and magnesium inside the cell so that this gradient is available to do work, and if there isn’t enough energy — affected by B12’s role in the Krebs cycle, then the cells can’t do this work and maintain the gradient of more K and Mg inside the cell).

When you are feeling run-down, then the main thing you have to worry about is B12 deficiency. All of these other deficiencies aren’t the critical factor so you don’t think about them.

However once you have sorted out the B12 deficiency, then the next problem could show itself. For example, with heart palpitations, once all the cells are working more-or-less normally then the K and MG imbalance starts to be important, and it will affect neurological-type tissue first. THe heart muscle is a hybrid nerve/muscle so it’s one of the tissues that gets very affected by this imbalance and you may get funnny effects, the most worrying of which is palpitations (often accompanied by nausea).

To cure it: I recommend that everyone in developed countries or with a westernised diet should take a multivitamin-multimineral daily supplements. Why not each individual vitamin and mineral separately? partly because there are so many of them, and mainly because the big cost in preparing vitamin supplements is the cost of separating one from another, whereas the manufacturer hasn’t had to do that with a multi-vitmin multi-mineral supplement so they are generally fairly cheap. This gets you your K, Mg, Vitamin D and a whole lot of trace stuff which may have absorbed badly for a few years whilst you had the B12 deficiency.

In the meantime, you may feel that you are going to die. You probably aren’t. THat doesn’t stop you calling for an ambulance but most people who report heart palpitations say it lasts less than 90 mins, and some people do report that it happens after each of their first fe injections, and sometimes after the 3 monthly injection for a while. Really, the best solution is to restore your body B12 levels until you don’t get it anymore. See below.

Should I stop taking B12 supplements because of this?

Will the symptoms go away if you stop taking the supplements? Yes probably. That’s why we call it «reversing out syndrome», because it’s like reversing through the symptoms you experienced when you were becoming B12 deficient.

HOWEVER if you stop taking B12 supplements, then you will stay the sick side of the symptoms, so the next time you take some B12 you could move into the Reversing Out barrier and get all the symptoms again, and in the meantime you haven’t improved.

What you really want to do is come out the other side. It’s like being stuck underwater with a great layer of rubbish on the surface — in order to breathe you need to push right through the layer of rubbish, but it can feel easier just to sink back underwater to get clear of the rubbish, even though you really want your life back.

So you need to keep going with the supplements, keep tolerating the symptoms, until you come out the other side. No, it isn’t the B12 that’s causing the symptoms, the B12 makes it so you can feel those symptoms again which is a big improvement on feeling nothing and there is a sky and oxygen on the other side (to stick with the swimming metaphor).

Susan gets very tired, which isn’t much fun especially as she has so much to do. Here she tells her story.

Just the way it works for humans, oral B12 can make a world of a difference for your pet. My old cat Smudge chases the young cat again, climbs ladders (and climbs down herself), and is generally as fit as she was many years ago.

June describes her suffering when doctors didn’t follow the standard protocol after any stomach or intestinal operation — to offer B12 replacement therapy

Frankie tells of how she suffered, the tests she had to endure until doctors worked out what was wrong, and what a difference it has made.

The Scottish Parliament discusses Pernicious Anaemia and vitamin B12 deficiency on Wednesday 7 March 2012.
This video is over 1 hour long and represents real political change — we are at last on our way.

Donna, like so many women, wants to live a normal life. Vitamin B12 could give her that chance.

Dr Chandy was nominated for the North East Local Heroes award. The interviewer was at first surprised — people don’t get awards for doing what they are paid to do — but she persisted.

New documentary out from Elissa Leonard in North America, featuring Sally Pacholok and many other internationally renowned experts.

Julia found her eyesight going as her eyes refused to focus on the same things as each other. She’s been for all sorts of tests and treatment, but now that she’s on B12 replacement therapy she’s starting to see an improvement.

The definitive and original guide to B12 deficiency, Dr Chandy interviewed by Chris Jackson of UK BBC Inside Out Team broadcast 31 Oct 2008.
Dr Joseph Chandy explains symptoms and shows the restorative effect on one patient (other patients’ families have asked that we edit out their stories unfortunately)

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