Most healthy adults who eat a varied diet don’t need a daily supplement, but some people clearly do: vitamin D in winter or with little sun, folic acid before and in early pregnancy, B12 for vegans and long-term metformin users, and iron when blood tests show it is low. This guide covers who benefits, sensible doses and upper limits, the popular extras (K2, CoQ10, omega-3, magnesium), interactions with medicines and how to judge a label.
The short answer
- Worth taking for specific groups: vitamin D (10–25 mcg / 400–1,000 IU a day with little sun), folic acid 400 mcg around conception, B12 for vegans and some medicine users, iron when deficiency is confirmed.
- More is not better: vitamin D above 100 mcg (4,000 IU) a day, vitamin A in pregnancy and high-dose iron can cause harm.
- Tell your pharmacist about supplements: K2 opposes warfarin, St John’s wort weakens the pill, and biotin can falsify blood tests.
Who actually needs a supplement?
| Group | Supplement | Typical amount |
|---|---|---|
| Everyone in autumn and winter (UK advice); people who rarely go outside, cover up, or have dark skin | Vitamin D | 10 mcg (400 IU) daily; more on medical advice if levels are low |
| Planning pregnancy until 12 weeks pregnant | Folic acid | 400 mcg daily (5 mg if higher risk, on prescription) |
| Pregnancy (Australia, many other countries) | Iodine | 150 mcg daily |
| Vegans; long-term metformin or acid-reducer users; over-60s with low levels | Vitamin B12 | Varies; check levels |
| Confirmed iron deficiency (heavy periods, pregnancy, low intake) | Iron | As prescribed; often alternate-day dosing is better tolerated |
| Babies and young children (UK) | Vitamins A, C and D drops | As per national programme |
Outside these groups, large trials have found little benefit from multivitamins for preventing heart disease, cancer or dementia in well-nourished adults. They do no harm at standard doses, but a varied diet matters more.
Vitamin D and K2
Vitamin D helps absorb calcium and keeps bones and muscles healthy. The skin makes it from sunlight, so needs depend on season, latitude, skin colour and how much skin is exposed. D3 (cholecalciferol) is the usual supplement form. The safe upper limit for adults is 100 mcg (4,000 IU) a day unless a doctor prescribes more for a measured deficiency.
Vitamin K2 (menaquinone, mostly as MK-7) is marketed with D3 to “direct calcium to bones, not arteries”. The idea is plausible, but trials so far show little clear benefit for fractures or heart disease. K2 is a real problem for people on warfarin, because any vitamin K opposes it. See vitamin D3 with K2: who needs it?
CoQ10, omega-3 and krill oil
Coenzyme Q10 is made by the body and used in energy production. Statins lower its levels, which led to the idea that CoQ10 could ease statin muscle aches. Trials are mixed: some people report benefit, but overall evidence is weak. Doses of 100–200 mg a day are well tolerated. CoQ10 comes as ubiquinone (the standard form) or ubiquinol (the reduced form, more expensive); see Spring Valley CoQ10: ubiquinol or ubiquinone?
Omega-3 fatty acids (EPA and DHA from fish, krill or algae oil) lower high triglycerides at prescription doses. For general heart protection in people without heart disease, large trials such as VITAL and ASCEND found little benefit, and high doses slightly increase the risk of atrial fibrillation. Krill oil has the same omega-3s in smaller amounts, bound to phospholipids, at a higher price. CoQ10 and fish or krill oil can be combined: CoQ10 and krill oil together.
Magnesium, calcium and iron
| Mineral form | Elemental content | Notes |
|---|---|---|
| Magnesium oxide | High (about 60%) | Cheap; poorly absorbed; laxative effect |
| Magnesium citrate | About 16% | Better absorbed; can loosen stools |
| Magnesium glycinate (bisglycinate) | About 14% | Gentle on the gut; popular for cramps and sleep, though evidence is limited |
| Calcium carbonate | 40% | Take with food |
| Calcium citrate | 21% | Absorbed with or without food |
| Ferrous sulfate / fumarate | About 20% / 33% | Effective; constipation and dark stools are common |
Check the elemental amount on the label: “500 mg magnesium citrate” contains only about 80 mg of magnesium. For adults, the upper limit for magnesium from supplements is about 350 mg a day; more causes diarrhoea.
Supplements that interact with medicines
| Supplement | Medicine affected | Problem |
|---|---|---|
| Vitamin K (incl. K2, green-leaf powders) | Warfarin | Reduces its effect; keep intake consistent and tell your clinic |
| St John’s wort | The pill, warfarin, SSRIs, ciclosporin, many others | Weakens them or adds serotonin effects |
| Calcium, iron, magnesium, zinc | Levothyroxine, doxycycline, ciprofloxacin, bisphosphonates | Block absorption: separate by 2–4 hours |
| Biotin (hair, skin, nail products) | Blood tests (thyroid, troponin, hormones) | False results; stop a few days before tests and tell the lab |
| Fish oil, ginkgo, garlic, turmeric (high dose) | Blood thinners, NSAIDs | Small extra bleeding risk |
| Potassium supplements | ACE inhibitors, ARBs, spironolactone | Dangerously high potassium |
| Vitamin A (retinol) | Pregnancy, isotretinoin | Birth defects at high doses; avoid high-dose A and liver oils in pregnancy |
How to judge a supplement label
- Regulation differs: in Australia, “AUST L” products are listed with the TGA but their efficacy isn’t evaluated, while “AUST R” products are assessed. In the US, supplements don’t need FDA approval before sale. In the UK, herbal products with the “THR” mark meet quality standards.
- Third-party testing (USP Verified, NSF Certified, Informed Sport) checks that the contents match the label.
- Look for the amount per dose in micrograms or milligrams, and the elemental amount for minerals, rather than “proprietary blends”.
- Be wary of claims to cure, detox, “boost immunity” or replace medicines, and of products sold mainly through testimonials.
- Compare price per effective dose, not per bottle.
Articles in this guide
Each article answers one specific question in depth.
- Can You Take CoQ10 and Krill Oil Together?CoQ10 and krill oil don't interact and are fat-soluble, so take them with a meal. Doses, statin users, blood thinners and what the evidence supports.
- Is Spring Valley CoQ10 Ubiquinol or Ubiquinone?Spring Valley sells both forms: standard CoQ10 softgels are ubiquinone, the 'Ubiquinol' line is ubiquinol. How to read the label and which form to choose.
- Vitamin D3 with K2 (MK-7): Who Needs It?D3 + K2 is marketed for bones and arteries. What the evidence shows, sensible doses, the warfarin warning and how to pick a product.
Frequently asked questions about supplements
What supplements should I take daily?
There is no list that suits everyone. Most adults in northern countries benefit from vitamin D in winter, people planning pregnancy need folic acid, and vegans need B12. Others are best guided by diet and, where relevant, a blood test.
Can I take vitamins with my medicines?
Usually yes, but minerals (calcium, iron, magnesium, zinc) need spacing from some medicines, and vitamin K and St John’s wort interact with several. Show your pharmacist the bottle.
Is it better to take vitamins in the morning or at night?
Fat-soluble vitamins (A, D, E, K), CoQ10 and fish oil are absorbed better with a meal containing fat. Iron is best absorbed on an empty stomach but is often taken with food to reduce nausea. Otherwise, take them when you’ll remember.
Do gummy vitamins work?
They can, but they often contain less of each nutrient, include sugar, and tend to lose potency faster. Check the amounts per serving and keep them away from children, as they look like sweets.
Sources and further reading
Combining medicinesWhich medicines clash and why: duplicate ingredients, NSAID combinations, sedatives, alcohol and blood pressure…
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