Nutrition · 8 March 2026 · 11 min read

Best Supplements to Take on Semaglutide UK: What You Actually Need (2026)

When you eat less on GLP-1, you absorb fewer vitamins and minerals. This guide covers the supplements that matter most — why you need them, what to look for, and the best UK options.

Last updated: March 2026 | 11 min read

When you start Ozempic, Wegovy or Mounjaro, your food intake drops significantly. That's the point — but it creates a real problem. When you eat less, you absorb fewer vitamins and minerals. Add in the fact that GLP-1 medications slow gastric emptying and can affect how your body absorbs certain nutrients, and you've got a recipe for deficiency if you're not careful.

This guide covers the supplements that matter most when you're on semaglutide or tirzepatide — why you need them, what to look for, and the best UK options available right now.

Do You Definitely Need Supplements on Semaglutide?

Not everyone will develop deficiencies. But the risk is real enough that most nutrition experts recommend at least a basic supplement routine for anyone on GLP-1 medication long term.

The main reasons your nutrient levels can drop:

• You're eating 30–50% less food overall, so you're getting fewer vitamins and minerals from your diet
• GLP-1 medications slow gastric emptying, which can affect absorption of some nutrients
• A low-fat diet (often recommended on GLP-1s) reduces absorption of fat-soluble vitamins A, D, E and K
• If you're eating less animal protein, B12 is at particular risk

The good news is that a simple, well-chosen supplement routine covers most of the gaps. You don't need an expensive stack — just a few key products taken consistently.

The Supplements Worth Taking on Semaglutide

1. Vitamin B12 — Most important supplement on GLP-1

B12 is the nutrient most at risk on semaglutide. GLP-1 medications affect stomach acid production and gastric emptying, both of which play a role in B12 absorption. If you're also eating less meat, fish, eggs and dairy — the main food sources of B12 — your levels can drop quickly.

Low B12 causes fatigue, brain fog, tingling in the hands and feet, and in serious cases, nerve damage. Many GLP-1 users report tiredness that they put down to the medication — in some cases it's actually low B12.

What to look for: Choose a methylcobalamin form of B12 (not cyanocobalamin) as it's better absorbed. A daily dose of 1000mcg is standard.

2. Vitamin D — Essential for bone and immune health

Vitamin D deficiency is already very common in the UK — our climate simply doesn't give us enough sunlight to produce it naturally, especially from October to March. People with higher BMIs are at even greater risk because vitamin D gets stored in body fat rather than circulating in the bloodstream.

On semaglutide, a low-fat diet reduces absorption of fat-soluble vitamins including D. Bone health matters especially during rapid weight loss, when the skeleton is under stress.

The NHS recommends everyone in the UK considers a vitamin D supplement from October to March. On GLP-1 medication, taking it year-round makes good sense.

What to look for: Vitamin D3 (cholecalciferol) rather than D2. A daily dose of 1000–2000IU is safe for most adults. Take with a meal containing some fat to improve absorption.

Recommended: BetterYou Vitamin D3 1000IU Daily Oral Spray — absorbs through the mouth rather than the stomach, which makes it ideal for GLP-1 users where gastric absorption can be affected.
Check price on Amazon UK (affiliate link)

3. Omega-3 Fish Oil — For heart health and inflammation

GLP-1 medications have proven cardiovascular benefits, but omega-3 fatty acids work alongside these to reduce inflammation, support heart health, and improve insulin sensitivity. Most UK adults don't get enough omega-3 from their diet, and eating less food on semaglutide makes this gap wider.

Omega-3 also supports brain health and mood — both relevant for GLP-1 users who sometimes report low mood or brain fog, particularly in the early weeks of treatment.

What to look for: Look for a fish oil with at least 500mg combined EPA and DHA per daily capsule. For vegetarians and vegans, algae-based omega-3 is an excellent alternative.

Recommended: Seven Seas Omega-3 Max Strength — trusted UK brand, widely available, good value for daily use.
Check price on Amazon UK (affiliate link)

4. Magnesium — For sleep, muscle function and constipation

Magnesium is one of the most commonly overlooked supplements, and GLP-1 users have particular reason to take it. First, eating less means taking in less dietary magnesium. Second, constipation is one of the most common side effects of GLP-1 medications — and magnesium has a gentle laxative effect that can help keep things moving.

Magnesium also supports muscle function (important when you're trying to preserve muscle mass), better sleep, and reduced fatigue.

What to look for: Magnesium glycinate or magnesium citrate are the best-absorbed forms. Avoid magnesium oxide, which is poorly absorbed and mainly used as a cheap filler. A daily dose of 200–400mg elemental magnesium is typical.

5. Iron — Especially important for women

GLP-1 medications may reduce iron absorption, and eating less meat (a key source of easily absorbed haem iron) increases the risk further. Iron deficiency is already the most common nutrient deficiency in the world, particularly in women of childbearing age.

Symptoms of low iron overlap with common GLP-1 side effects — fatigue, weakness, breathlessness — which means it's easy to miss. If you're experiencing persistent tiredness on semaglutide, it's worth asking your GP for a blood test to check iron and ferritin levels before supplementing.

What to look for: Iron bisglycinate is the gentlest form on the stomach (important on GLP-1, where nausea is already a risk). Standard iron supplements can cause constipation and nausea — the last thing you need.

Note: Don't take iron unless you know your levels are low. Too much iron is harmful. Ask your GP for a ferritin test first.

6. Multivitamin — The simple safety net

If you'd rather not take five separate supplements, a good quality multivitamin covers most of the bases in one tablet. It won't give you therapeutic doses of any individual nutrient, but it provides a solid safety net against the most common deficiencies.

Look for a multivitamin that includes B12, vitamin D, iron (if appropriate for you), zinc, and vitamin A.

Recommended: Healthspan MultiVitality Gold — comprehensive UK multivitamin, includes B12, D3, iron, zinc and more.
Check price on Amazon UK (affiliate link)

Supplements to Avoid on Semaglutide

Not everything is safe or helpful. A few things to be cautious about:

• Calcium supplements: GLP-1 medications may already raise calcium levels in some people. Don't take extra calcium unless your GP has confirmed you're deficient.
• Weight loss supplements: Products containing ephedra, green tea extract, garcinia cambogia or similar stimulants should not be combined with GLP-1 medications. They can worsen side effects and the interactions aren't well studied.
• Very high-dose fat-soluble vitamins: Vitamins A, D, E and K accumulate in body fat. Megadosing these can be dangerous. Stick to standard recommended amounts unless a GP has advised otherwise.
• Anything that claims to "boost" GLP-1 or "enhance semaglutide": These products are not clinically proven and may interfere with your medication. Ignore the marketing.

Should You Get a Blood Test First?

Ideally, yes. Before starting a supplement routine, a simple blood test can show your baseline levels of B12, vitamin D, iron (ferritin), and folate. Your GP can request this, or you can use a private home testing service.

This matters because:

• Supplementing nutrients you're not deficient in is wasted money
• Some deficiencies are serious enough to need prescription-strength treatment, not just supplements
• It gives you a baseline to compare against in 6–12 months

If you don't want to wait for a GP appointment, home blood test kits covering the key nutrients are available from Medichecks and similar services in the UK.

How to Take Supplements Alongside GLP-1 Medication

A few practical tips:

• Take supplements with food where possible — this improves absorption of fat-soluble vitamins and reduces the chance of nausea
• Space iron away from other supplements — iron blocks absorption of zinc and calcium if taken together
• If nausea is a problem, try B12 as a sublingual (dissolves under the tongue) tablet — it bypasses the stomach entirely
• Oral sprays for vitamin D (like BetterYou) are excellent for GLP-1 users as they absorb through the mouth, not the gut

Quick Summary: The Essential Semaglutide Supplement Stack

• Vitamin B12 (1000mcg methylcobalamin daily) — essential
• Vitamin D3 (1000–2000IU daily, year round in UK) — essential
• Omega-3 fish oil (500mg+ EPA/DHA daily) — highly recommended
• Magnesium glycinate (200–400mg daily) — recommended, especially for constipation
• Iron (only if blood test confirms deficiency) — as needed
• Multivitamin — optional but a good safety net if you prefer simplicity

Frequently Asked Questions

Can I take supplements on the same day as my injection?
Yes. There's no known interaction between GLP-1 medications and standard vitamin and mineral supplements.

Will supplements help with side effects?
Magnesium can help with constipation. B12 can help with fatigue if you're deficient. Omega-3 supports general wellbeing. But supplements won't eliminate nausea or other GLP-1 side effects directly.

Do I need supplements forever?
For as long as you're on GLP-1 medication and eating significantly less than usual, yes. Once you've stabilised at a healthy diet with good variety, a blood test can tell you whether you still need them.

Is it worth paying for expensive branded supplements?
Not always. For B12 and vitamin D, the key thing is the form of the nutrient (methylcobalamin B12, D3 not D2) rather than the brand. A well-reviewed, reasonably priced supplement from a UK-registered brand is absolutely fine.

This article contains affiliate links. If you buy via our links we may earn a small commission at no extra cost to you. GLP1Guide.live does not provide medical advice. Always consult your doctor or pharmacist before starting any new supplement, especially if you are on prescription medication.