Mounjaro, iron, B12 and vitamin D: nutrients to watch

In short

UK iron reference intake: 14.8mg a day for women aged 19 to 50, 8.7mg for men and women over 50
UK vitamin B12 reference intake: 1.5 micrograms a day for adults
The NHS says everyone should consider 10 micrograms (400 IU) of vitamin D a day from October to early March
Do not take more than 100 micrograms (4,000 IU) of vitamin D a day unless a doctor advises it
Metformin can lower B12 levels, and the MHRA advises B12 checks in people at risk
Tea and coffee with meals reduce iron absorption

Mounjaro does not block iron, vitamin B12 or vitamin D. Eating much less food does make all three harder to get, and the effect builds over months. Women aged 19 to 50 need 14.8mg of iron a day, adults need 1.5 micrograms of B12, and the NHS advises everyone to consider 10 micrograms of vitamin D from October to March. If you feel tired, breathless or get pins and needles, ask your GP for a blood test.

Why these three nutrients

None of iron, B12 or vitamin D deficiency appears in the Mounjaro SmPC. The concern is intake. Decreased appetite is very common, affecting more than 1 in 10 people, and many people eat half their usual amount for months. The foods people often go off, such as red meat, are some of the best sources of iron and B12.

Fatigue is itself a very common side effect of Mounjaro. Low iron and low B12 also cause tiredness, so a deficiency can hide behind what looks like an ordinary side effect. The fatigue page covers other causes.

UK daily amounts

The government dietary recommendations set these reference intakes for adults:

Nutrient	Men	Women 19 to 50	Women over 50
Iron	8.7mg	14.8mg	8.7mg
Vitamin B12	1.5 micrograms	1.5 micrograms	1.5 micrograms
Vitamin D	10 micrograms	10 micrograms	10 micrograms

These amounts meet the needs of almost everyone. Eating a little less on some days does no harm. Months of tiny meals do.

Iron

Who is most at risk

The NHS names heavy periods and pregnancy as very common causes of iron deficiency anaemia. On Mounjaro, risk is higher if you:

Have periods, especially heavy ones. Some people find periods change on Mounjaro. See Mounjaro and periods.
Eat little or no red meat
Follow a vegetarian or vegan diet
Take ibuprofen or aspirin regularly, which can cause bleeding in the gut

Signs of low iron

The NHS lists tiredness, breathlessness, palpitations, pale skin and headaches as the main symptoms. Less common signs include a sore tongue, altered taste, hair loss, mouth ulcers, restless legs and cravings for ice or other non-food items. Hair shedding on Mounjaro is common, and low iron makes it worse. See the hair loss page.

Getting more iron from small meals

Good sources, with typical amounts:

Fortified breakfast cereal, 30g: often 2mg to 4mg (check the label)
Lean beef mince, 100g cooked: about 2.5mg
Cooked lentils, 100g (about 4 tablespoons): about 2.5mg to 3mg
2 eggs: about 2mg
Tinned sardines, 100g: about 2.5mg
Dried apricots, 4 to 5: about 1mg to 1.5mg
Dark green vegetables such as kale or spinach

Iron from meat and fish absorbs more easily than iron from plants. Vitamin C helps the body take up plant iron, so add a glass of orange juice, a handful of berries or peppers to the meal. The NHS notes tea, coffee, milk and dairy can reduce absorption in large amounts. Drink tea and coffee between meals rather than with them.

Do not start iron tablets without a blood test. Iron tablets commonly cause constipation, nausea and stomach pain, all of which Mounjaro already causes.

Vitamin B12

Who is most at risk

The NHS lists pernicious anaemia, an autoimmune condition, as the most common cause of B12 deficiency in the UK. Diet is the other main cause, especially a vegan diet without fortified foods, or a restrictive or poor diet over a long period. Medicines also play a part:

Metformin. The MHRA advises checking B12 in people on metformin who have symptoms or risk factors. Many people with type 2 diabetes take metformin alongside Mounjaro.
Proton pump inhibitors such as omeprazole and lansoprazole, often taken for reflux, which is common on Mounjaro
Some anticonvulsants

Signs of low B12

The NHS lists extreme tiredness, pins and needles, a sore or red tongue, mouth ulcers, muscle weakness, vision problems, mood changes, and problems with memory and judgement. Some of the nerve damage can be permanent, so early testing matters.

Food sources

B12 comes from animal foods and fortified products:

Salmon, 100g: several times the daily amount
Lean beef, 100g: more than the daily amount
200ml of milk: about half the daily amount
1 egg: about a third to half the daily amount
Fortified breakfast cereals, yeast extract and plant milks (check the label)

If you are vegan, use fortified foods or a B12 supplement every day.

Vitamin D

What the NHS advises

From about late March to the end of September, most people in the UK make enough vitamin D from sunlight on their skin. Between October and early March the sun is not strong enough. The NHS advises everyone to consider a daily 10 microgram (400 IU) supplement in autumn and winter.

Take it all year round if you:

Rarely get outdoors, or live in a care home
Usually cover most of your skin when outside
Have dark skin, for example if you are of African, African-Caribbean or South Asian background

Do not take more than 100 micrograms (4,000 IU) a day unless a doctor advises it. Long-term excess raises calcium levels and can damage the kidneys, bones and heart.

Food sources

Few foods contain much vitamin D. The best are oily fish (salmon, sardines, mackerel, trout, herring), egg yolks, red meat and fortified foods such as some spreads and breakfast cereals. UK cows' milk is generally not fortified.

Vitamin D supports bone strength, which matters when you lose weight quickly. Weight loss reduces the load on bones, so keep up strength exercise and calcium-rich foods too.

Blood tests and supplements

If you have symptoms, your GP can arrange:

A full blood count, which shows anaemia
Ferritin, which shows iron stores before anaemia develops
Vitamin B12 and folate
Vitamin D, usually only if you have symptoms or risk factors

If you buy Mounjaro privately, tell your GP you are taking it. A standard multivitamin and mineral supplement at around 100% of daily amounts is a sensible safety net when meals are small. Avoid high-dose single nutrients unless a blood test shows you need them. Our guide to multivitamins and Mounjaro covers what to look for. If your meals have shrunk a long way, read eating too little on Mounjaro.

When to get help

Call 999 or go to A&E if:

You have chest pain, or severe breathlessness at rest
You are vomiting blood or passing black, tarry stools
You have fainted or collapsed

Call 111 or see a GP today if:

You feel breathless doing everyday things, or have a racing heartbeat
You have new numbness, pins and needles or weakness in your hands or feet
You have problems with balance, walking or vision
You have heavy periods with dizziness or feeling faint

Mention at your next check if:

You feel tired most days despite eating regular meals
Your hair is shedding more than usual
You take metformin or a proton pump inhibitor and have never had a B12 test
You follow a vegetarian or vegan diet and eat far less than before
You want advice on which supplements to take

Report side effects you think are linked to Mounjaro through the MHRA Yellow Card scheme.

Questions people ask

Does Mounjaro cause iron deficiency?

Mounjaro does not block iron absorption directly, and iron deficiency is not a listed side effect. The risk comes from eating much less, especially less red meat and fortified cereal. Women with periods, people who avoid meat and anyone eating very small meals for months are at higher risk.

Should I take vitamin D on Mounjaro?

The NHS advises everyone in the UK to consider 10 micrograms of vitamin D a day from October to early March, on or off Mounjaro. People who get little sun, cover their skin outdoors, or have dark skin should take it all year. Do not take more than 100 micrograms a day unless a doctor advises it.

What are the signs of low B12?

The NHS lists extreme tiredness, pins and needles, a sore or red tongue, mouth ulcers, muscle weakness, problems with vision or memory, and mood changes. Some nerve damage from B12 deficiency can be permanent, so see your GP early if you notice these.

Can I ask my GP for blood tests while on Mounjaro?

Yes, if you have symptoms. A GP can check a full blood count, ferritin (iron stores), B12, folate and vitamin D. If you buy Mounjaro privately, tell your GP you are taking it so they can interpret the results.

Should I take a multivitamin on Mounjaro?

A standard multivitamin and mineral at 100% of daily amounts is a reasonable safety net if your meals are very small. Avoid high-dose single supplements, especially iron, unless a blood test shows you need them. Too much iron causes constipation and nausea, both already common on Mounjaro.

Sources

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: Mounjaro, iron, B12 and vitamin D: nutrients to watch. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-iron-b12-and-vitamin-d/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

Mounjaro (tirzepatide) does not directly block iron, vitamin B12 or vitamin D, but eating much less food makes all three harder to get. UK guidance sets daily amounts of 14.8mg iron for women aged 19 to 50 and 8.7mg for men, 1.5 micrograms of B12, and 10 micrograms of vitamin D. The NHS advises everyone to consider a 10 microgram vitamin D supplement from October to March. Tiredness, breathlessness and pins and needles are signs to ask your GP for a blood test.

UK iron reference intake: 14.8mg a day for women aged 19 to 50, 8.7mg for men and women over 50
UK vitamin B12 reference intake: 1.5 micrograms a day for adults
The NHS says everyone should consider 10 micrograms (400 IU) of vitamin D a day from October to early March
Do not take more than 100 micrograms (4,000 IU) of vitamin D a day unless a doctor advises it
Metformin can lower B12 levels, and the MHRA advises B12 checks in people at risk
Tea and coffee with meals reduce iron absorption

Cite as: Mounjaro Info, "Mounjaro, iron, B12 and vitamin D: nutrients to watch", https://mounjaroinfo.co.uk/side-effects/mounjaro-iron-b12-and-vitamin-d/, reviewed 22 October 2026.