Vitamin and Medicine Interactions: A Reference Table (Blood Thinners, Statins, Metformin, Thyroid and More)
A reference table of documented vitamin and medicine interactions, covering blood thinners, statins, metformin, thyroid medicine, acne medicine and more, with sources and gaps.
Content last updated . This page has not been medically reviewed.
How to use this table
Vitamins can change how a medicine works, and some medicines lower vitamin levels. This page lists the interactions found in the sources read, and where a pairing is not covered it says so. It is information only: do not stop or change a prescribed medicine because of anything here. Tell your prescriber and pharmacist about every vitamin and supplement you take, as NHS medicine pages ask, and ask them what is right for you. [1][2]The last column says what kind of source supports each row (official medicine information, trial, case reports) or insufficient evidence where the sources read did not answer the question. A missing row does not mean "no interaction"; it may only mean it was not covered by the sources. See also How interactions work and Interactions by drug class.
Reference table
| Vitamin | Medicine or test | Concern | Kind of source |
|---|---|---|---|
| Vitamin K (supplements, and big changes in vitamin K foods) | Warfarin (blood thinner) | The NHS says not to take vitamin K supplements while on warfarin, and that vitamin K-rich foods can change how warfarin works [1]. MedlinePlus says sudden changes in vitamin K intake can change the medicine's effect [3] | Documented interaction; NHS 2026, MedlinePlus 2025 |
| Vitamin E (high-dose supplements) | Warfarin and other blood thinners | High-dose vitamin E has been linked to cases of stronger anticoagulant effect, and a trial of 1,000 IU a day for 12 weeks found a marker of poorer vitamin K status in people not on anticoagulants [4]. MedlinePlus says high doses might raise the risk of bleeding in the brain [5] | Case reports and small trials; not a proven rate |
| Niacin (nicotinic acid, high prescription-type amounts) | Statins | In HPS2-THRIVE (25,673 high-risk patients on statin-based treatment, median 3.9 years), extended-release niacin with laropiprant, 2,000/40 mg a day, added no cardiovascular benefit [6] and raised definite myopathy (muscle disease) risk, risk ratio 4.4 [7]. This is a study amount of a prescription combination, not advice and not shown for over-the-counter niacin | Trial evidence |
| Calcium and iron supplements | Levothyroxine (thyroid hormone) | The NHS lists calcium and iron supplements among things that may not mix well with levothyroxine and says to take it 30 to 60 minutes before the first meal [2]. Timing is for your prescriber or pharmacist to set | Documented interaction; NHS 2026 |
| Biotin (vitamin B7) | Thyroid blood tests (the medicine itself is not changed) | The NHS lists biotin because it can affect thyroid tests [2]. The FDA says biotin can interfere with certain lab tests and give incorrect results that may go undetected, including falsely low troponin results [8] | Lab-test interference; see Biotin |
| Vitamin B12 (low levels) | Metformin | The NHS lists vitamin B12 deficiency as a common side effect of metformin [9]. Whether to check B12 is a clinician's decision | Documented side effect |
| Vitamin B12 (very low levels) | Omeprazole (acid reducer) | The NHS lists a very low vitamin B12 level as a possible serious side effect [10] | Documented side effect |
| Vitamin A supplements | Isotretinoin (acne medicine) | The NHS lists vitamin A supplements, and supplements that contain it, among things that may not mix well [11] | Documented interaction |
| Folic acid | Methotrexate | The NHS says folic acid may be prescribed with methotrexate, and that taking it on the same day can stop the medicine working properly. Follow the prescriber's schedule exactly [12] | Documented, prescriber-directed |
| Fat-soluble vitamins (A, D, E, K) | Orlistat (weight-loss medicine) | Orlistat works by reducing fat absorption [13], and fat-soluble vitamins are absorbed with dietary fat [14]. Proper evidence not found in the pages read on how much orlistat changes vitamin levels, so ask the prescriber | Insufficient evidence in sources read |
| Folic acid at high amounts | Vitamin B12 deficiency (not a medicine) | The NHS says folic acid above 1 mg a day can mask vitamin B12 deficiency symptoms [15] | Documented |
What the sources did not cover
- Vitamin D and thyroid medicine: the NHS levothyroxine page names calcium, iron and biotin, not vitamin D. Proper evidence not found in the sources read that vitamin D changes levothyroxine; calcium-plus-D products contain calcium, so read the label. [2] See Calcium and vitamin D products.
- Other statins and vitamins (C, E, A, D): searches of NHS, MedlinePlus and PubMed for this page found trial evidence only for niacin. Proper evidence not found for the rest in the sources read.
- Newer blood thinners: the NHS warfarin page covers warfarin only. Proper evidence not found here for vitamins with newer anticoagulants; ask your clinic. Warfarin details are on Vitamins and blood thinners.
- Chemotherapy and radiotherapy: see Vitamins during cancer treatment.
Pregnancy, children, kidney, liver and surgery
Interactions matter more with many medicines, with kidney or liver disease, around surgery, and in pregnancy. Warfarin, for example, is not usually used in pregnancy and may not suit people with liver or kidney problems or those about to have surgery, per the NHS. [1] Read Vitamins before surgery and Vitamins and kidney or liver disease. Product quality also affects what you are really taking: Quality testing and certifications.Frequently asked questions
Can I take vitamin K if I am on warfarin?
The NHS says not to take vitamin K supplements while on warfarin, and to talk to your warfarin clinic about vitamin K foods. [1] Do not change your warfarin yourself.
Does metformin cause low vitamin B12?
The NHS lists vitamin B12 deficiency as a common side effect, which can make you feel very tired. [9] Ask your clinician whether to check your level; do not start or stop anything on your own.
Can I take calcium or iron with thyroid medicine?
The NHS says calcium and iron supplements may not mix well with levothyroxine. [2] Your pharmacist can advise on spacing for your prescription.
Does biotin affect thyroid or heart tests?
It can interfere with some lab tests, including thyroid tests and troponin, according to the NHS and the FDA. Tell the clinician and the lab you take biotin. [2][8]
Is niacin safe with a statin?
A large trial of a prescription niacin-laropiprant combination on top of a statin found more muscle problems and no cardiovascular benefit. [6][7] Ask your prescriber before using niacin in any form.
References
- NHS (UK), Warfarin, page last reviewed 24 February 2026. Read 2026-10-23.
- NHS (UK), Levothyroxine, page last reviewed 6 July 2026. Read 2026-10-23.
- MedlinePlus Medical Encyclopedia, Vitamin K, review date 21 January 2025. Read 2026-10-23.
- Effect of vitamin E supplementation on vitamin K status in adults with normal coagulation status. Am J Clin Nutr 2004. PubMed abstract, read 2026-10-23.
- MedlinePlus Medical Encyclopedia, Vitamin E, review date 21 January 2025. Read 2026-10-23.
- Serious adverse effects of extended-release niacin/laropiprant: results from HPS2-THRIVE. Clin Ther 2019. PubMed abstract, read 2026-10-23.
- HPS2-THRIVE randomized placebo-controlled trial in 25,673 high-risk patients of ER niacin/laropiprant: muscle and liver outcomes. Eur Heart J 2013. PubMed abstract, read 2026-10-23.
- US FDA, Biotin Interference with Troponin Lab Tests - Assays Subject to Biotin Interference. Read 2026-10-23.
- NHS (UK), Metformin, page last reviewed 5 August 2026. Read 2026-10-23.
- NHS (UK), Omeprazole, page last reviewed 17 September 2025. Read 2026-10-23.
- NHS (UK), Isotretinoin capsules, page last reviewed 26 June 2026. Read 2026-10-23.
- NHS (UK), How and when to take methotrexate, page last reviewed 14 March 2023. Read 2026-10-23.
- NHS (UK), Orlistat, page last reviewed 6 May 2026. Read 2026-10-23.
- MedlinePlus Medical Encyclopedia, Vitamins, review date 21 January 2025. Read 2026-10-23.
- NHS (UK), B vitamins and folic acid, page last reviewed 3 August 2020. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.