Safety tool
Can I take this supplement with my medicine?
A handful of supplement and medicine combinations are worth checking before you combine them, especially if you take blood pressure, thyroid, anticoagulant, hormone or diabetes medication. The table below lists common pairings against BNF and NHS guidance. If a combination is not listed, that means we have not found authoritative UK guidance for it, not that it is safe; ask a pharmacist. Information only, not medical advice; always check a new combination with your GP or pharmacist.
Common pairings to check
| Supplement | Medicine | What to know | Source |
|---|---|---|---|
| Vitamin B12 | Metformin | Ask a pharmacist Long-term metformin use is linked to reduced vitamin B12 absorption. Periodic B12 level checks are standard practice for people on long-term metformin, so mention any B12 supplement to your GP or pharmacist. | BNF, metformin hydrochloride |
| Potassium supplements | Spironolactone | Caution Spironolactone is potassium-sparing. Taking potassium supplements alongside it raises the risk of hyperkalaemia (high blood potassium). The same caution applies to salt substitutes that contain potassium. | BNF, spironolactone |
| St John's Wort | HRT or combined hormonal contraception (pill, patch, ring) | Caution St John's Wort induces liver enzymes that can reduce the effectiveness of hormone treatments, including HRT and the combined contraceptive pill, patch and ring. The MHRA has issued safety guidance on this interaction. | MHRA / GOV.UK, medicine safety information |
| Calcium or iron supplements | Levothyroxine | Space doses apart Calcium and iron can reduce the absorption of levothyroxine if taken too close together. The standard advice is to separate dosing by at least four hours. | BNF, levothyroxine sodium |
| Vitamin K | Warfarin | Caution Vitamin K can reduce the anticoagulant effect of warfarin, so a sudden change in vitamin K intake, whether from food or a supplement, can affect INR control. | BNF, warfarin sodium |
| Magnesium supplements | Blood pressure or heart rhythm medication | Ask a pharmacist Magnesium can add to the blood-pressure-lowering effect of some antihypertensives, and very high supplemental doses can affect heart rhythm medicines. Anyone on blood pressure or heart medication should check with a pharmacist before adding a magnesium supplement. | NHS, vitamins and minerals (magnesium) |
Not listed here? That means we have not found authoritative UK guidance for that specific combination, not that it is confirmed safe. Ask a pharmacist, who can check the full BNF interactions data for your exact medicines.
How we built this
Each row is drawn from the BNF (British National Formulary) drug monograph for the medicine, cross-checked against the equivalent NHS medicines page where one exists, and against MHRA safety guidance for the St John's Wort entry. We only include a pairing where we found a clear, named UK source for it. Where we have not verified a specific deep link on a source's site, we link to that organisation's general medicines or interactions page instead of guessing a URL. If a combination you are wondering about is not in the table, treat that as an open question, not a green light, and ask a pharmacist; a pharmacist has access to the full BNF interactions checker for every medicine and dose, which this page does not attempt to replace. Reference data as of 2026-08-04.
Information only, not medical advice. This tool is educational and is not a substitute for a registered clinician. It does not diagnose anything and does not recommend that you take any supplement. Always read product labels and speak to your GP or pharmacist before starting, stopping or combining supplements. This page covers a small, illustrative set of well-documented pairings. It is not a complete interaction checker and does not know your dose, your other medicines or your health history. Always check a new supplement against your current prescribed medicines with your GP or pharmacist before starting it.
Read more
See our guide to supplements for menopause, the evidence and dosing notes on magnesium, and the upper safe limit checker for how much of a nutrient is too much. More tools are on the tools hub.
Frequently asked questions
Can I take magnesium with blood pressure medication?
Magnesium can add to the blood-pressure-lowering effect of some antihypertensives, and very high supplemental doses can affect heart rhythm medicines. If you take blood pressure or heart medication, check with a pharmacist before adding a magnesium supplement. Source: NHS, vitamins and minerals (magnesium).
Does St John's Wort affect the contraceptive pill or HRT?
Yes. St John's Wort induces liver enzymes that can reduce the effectiveness of hormone treatments, including HRT and the combined contraceptive pill, patch and ring. The MHRA has issued safety guidance on this interaction, so it is not something to combine without checking first. Source: MHRA/GOV.UK safety information and the BNF St John's Wort interactions entry.
Why do I need to space out calcium and levothyroxine?
Calcium and iron can reduce how much levothyroxine your body absorbs if taken too close together. The standard advice is to separate dosing by at least four hours. Source: BNF, levothyroxine sodium monograph, and the NHS levothyroxine medicine page.
Can I take vitamin B12 with metformin?
Long-term metformin use is associated with reduced vitamin B12 absorption, which is why periodic B12 level checks are standard practice for people on long-term metformin. A B12 supplement is not automatically a problem, but mention it to your GP or pharmacist, particularly if you have been on metformin for some time. Source: BNF, metformin hydrochloride monograph, and the NHS metformin medicine page.
Editor, Her Vitals
Oliver leads Her Vitals's editorial coverage of women's life-stage health and supplements. He curates and reviews existing branded products across trying to conceive, pregnancy, postnatal, perimenopause, menopause and the senior years, weighing what the evidence supports against guidance from bodies such as EFSA, the NHS and NICE, and is clear that the content is information rather than medical advice.
Last reviewed: 4 August 2026
Reviewed by Oliver Mackman, Editor. Editorial review against published guidance; not a clinical review.