Vitamin B12: tablets, injection or IV drip

What vitamin B12 does and how to tell if you are low

According to the NHS (National Health Service), the UK’s health service, a lack of B12 can cause general symptoms such as tiredness and weakness, shortness of breath, palpitations, headache, loss of appetite, a red or sore tongue and problems with memory or concentration, and neurological symptoms such as numbness, tingling, muscle weakness and balance problems.

B12 also affects mental health. The UK clinical guideline NICE (2024) notes that deficiency can be linked to symptoms of depression, anxiety or, less often, psychosis.

Symptoms usually appear slowly. Diagnosis and treatment should be timely, because some neurological damage may not reverse if the deficiency is left untreated for a long time.

What blood tests usually cover when looking into tiredness and B12

• full blood count;
• vitamin B12 and folic acid;
• ferritin (iron stores);
• thyroid function;
• homocysteine or methylmalonic acid, when the B12 result is unclear or symptoms do not fit.

A normal B12 result does not always rule out deficiency. This happens, for example, with nitrous oxide use (known as “laughing gas”): B12 may look normal in the blood but not work. If this applies to you, tell the doctor, because it changes which tests are needed.

Why people run low on B12: the most common causes

B12 is found in meat, fish, eggs and dairy. To absorb it, the stomach makes a protein called intrinsic factor, and absorption finishes at the end of the small intestine. So deficiency can come from diet or from an absorption problem. The NHS and NICE mention:

• Pernicious anaemia (autoimmune gastritis). An autoimmune disease in which the body stops making intrinsic factor. It is the most common cause in the UK and has nothing to do with diet.
• A vegan diet or very few animal products, without supplements or fortified foods.
• Stomach or bowel surgery, for example a gastrectomy, some weight-loss operations or removal of the end of the small intestine.
• Bowel diseases, such as Crohn’s disease or coeliac disease.
• Medicines and substances: stomach protectors (such as omeprazole) and metformin for diabetes, especially long term, and nitrous oxide, both in anaesthesia and in recreational use.

Worth knowing: the liver stores B12 for several years. If you changed your diet recently, deficiency may take a long time to appear.

Tablets, injection or drip: the difference

Route	How it is given	When the doctor usually chooses it	What to know
Tablets	By mouth, daily	Deficiency from diet or the medicines mentioned, and in some mild intestinal absorption problems	Consistency is key
Intramuscular injection	A shot into the muscle	Pernicious anaemia, surgery on the stomach or end of the bowel, neurological symptoms or when stores need replacing quickly	Always under medical supervision
IV drip	Into a vein over 30-90 minutes	Not the usual treatment for B12 deficiency	The doctor may consider adding B12 to a drip prescribed for another reason

According to NICE (2024), in pernicious anaemia, after total removal of the stomach or of the end of the small intestine, B12 is replaced with lifelong injections, not tablets.

In Spain, your doctor decides the exact schedule, meaning how often the injection is given.

According to the Spanish Medicines Agency (AEMPS), 1,000-microgram vitamin B12, both as an injection and as tablets, requires a doctor’s indication. In other words, B12 at these doses is not something to inject or take on your own “for energy”.

Do tablets work as well as injections?

In practice, high-dose tablets are a real option for many people, especially when the deficiency comes from diet or a medicine. Injections remain the choice when absorption is badly affected, when there are neurological symptoms or when stores need replacing quickly. The doctor decides based on your blood test and your history.

Side effects and precautions

Injectable B12 is generally well tolerated. Possible side effects are pain, swelling or itching at the injection site, nausea, diarrhoea, headache, dizziness and hot flushes. Severe allergic reactions are rare but do occur, which is why injections and drips are given by healthcare staff.

Three precautions worth knowing:

• Folic acid can mask a B12 deficiency. If anaemia is treated with folic acid alone, symptoms may improve while nerve damage continues. That is why the doctor checks B12 before prescribing folic acid.
• Supplements affect blood tests. If you already take B12 as tablets, injections or vitamin complexes, tell the doctor before the test: the result may be normal even though the deficiency has not been corrected.
• Your regular medicines matter. Tell the doctor if you take omeprazole, metformin or other medicines on an ongoing basis.

B12 at home in Madrid with Sensor23 HOME

At Sensor23 HOME the doctor may consider including vitamin B12 in a drip prescribed after a prior consultation. This is how it works:

1. Medical consultation. The doctor reviews your history, blood tests, medicines and possible contraindications. If you have no recent blood test, the doctor will tell you which one to have first.
2. Treatment plan. If a drip is medically justified, the doctor decides its composition and whether it includes B12.
3. Nurse visit. A registered nurse comes to your home, hotel or office in Madrid. The drip takes 30 to 90 minutes.

What	Price in Madrid
Vitamin therapy (individual drip based on blood tests)	from €339
Vitamin B12 1 mg as a drip add-on, if the doctor prescribes it	€25

If your doctor has already prescribed B12 injections, they are usually given by the nurses at your health centre. If you have questions about the schedule, a Sensor23 doctor can go through it with you.

What to do if you think you are low on B12

• First, a blood test: full blood count, B12, folic acid, ferritin and thyroid.
• If it comes back low or unclear, the doctor looks for the cause: diet, stomach, bowel, medicines or nitrous oxide.
• Depending on the cause, the doctor prescribes tablets or injections, and a follow-up blood test.
• A drip only if the doctor prescribes it for another reason; the doctor may consider including B12.