Medicines and Nutrients: A GP's Guide to Spacing, Absorption and Long-Term Prescriptions
Written by Dr Mark Wilshere and Dr Marc Crutchley, GMC-registered GPs and founders of Dr Marks.
Medically reviewed by Dr Mark Wilshere MBChB MRCGP DCH BSc (Hons) (GMC no. 6144605). Last reviewed: 24 October 2026.
Important clinical notice before you read on
- Never stop, reduce or change a prescribed medicine without speaking to your GP or pharmacist first.
- Test, don't guess: if you take a long-term medicine (such as metformin or omeprazole) and develop symptoms such as ongoing tiredness, pins and needles or muscle twitches, ask your GP whether a blood test is appropriate. A food supplement is not a substitute for a blood test or prescribed treatment for a diagnosed deficiency.
- Always check with your pharmacist before starting any supplement alongside regular prescriptions, especially anticoagulants (such as warfarin), thyroid medicines, bone-protection tablets or antibiotics.
As practising UK GPs, two of the most common questions we are asked in clinic are: “Does my regular prescription affect my vitamin levels?” and “Can I take my daily supplement at the same time as my morning tablets?”
This hub brings together UK guidance from the NHS, the British National Formulary (BNF) and the Medicines and Healthcare products Regulatory Agency (MHRA) in one clear reference — along with links to our detailed GP guides and a quick-check ingredient table for the Dr Marks range.
1. When long-term medicines affect nutrient levels or intake
Most people taking regular medicines don't develop vitamin or mineral problems. However, UK regulators and clinical guidance highlight a few widely prescribed medicine groups where nutrient levels or overall dietary intake can drop over time:
| Medicine group | Nutrients of interest | What UK guidance (MHRA / NHS) says | Our detailed GP guide |
|---|---|---|---|
|
Metformin (for type 2 diabetes) |
Vitamin B12 | In 2022, the MHRA updated its advice: reduced vitamin B12 is a common side effect of metformin (affecting up to 1 in 10 people), especially at higher doses or with longer use. Speak to your GP for a blood test if you develop extreme tiredness, pins and needles, or a sore red tongue. Keep taking metformin while levels are checked. | Metformin and vitamin B12: what you need to know → |
|
Proton pump inhibitors (PPIs) (omeprazole, lansoprazole, esomeprazole, pantoprazole) |
Magnesium & Vitamin B12 | The MHRA warns that long-term PPI use (usually over 1 year, especially alongside diuretics or digoxin) can cause low blood magnesium (hypomagnesaemia). The NHS omeprazole guide also notes that long-term use can lower vitamin B12. Ask your GP whether a blood test is needed if you have symptoms. | Supplements and common medicines: what not to mix → |
|
GLP-1 & GIP weight-loss medicines (semaglutide / Wegovy / Ozempic, tirzepatide / Mounjaro) |
General micronutrient & protein intake (Vitamin D, iron, B12, folate, calcium, zinc) | Because GLP-1 medicines work by significantly reducing appetite and portion sizes, total intake of vitamins, minerals and protein can fall alongside calorie intake. Severe or persistent vomiting also needs medical review (including for hydration and thiamine/B1). Prioritise protein at meals and discuss nutritional cover with your clinician. | The Skinny Jab Truth: A GP's guide to GLP-1 medications → |
2. Medicines and supplements that need spacing (or avoiding)
Minerals such as iron, calcium, magnesium and zinc can bind to certain tablets in the stomach and gut, reducing how much of your medicine is absorbed. In most cases, simply leaving a gap of a few hours solves the problem — while a few combinations (such as warfarin and vitamin K) should be avoided unless your clinic advises otherwise.
| Prescribed medicine | Nutrients / supplements to watch | BNF & NHS spacing & clinical rule |
|---|---|---|
|
Levothyroxine (underactive thyroid) |
Iron, calcium, magnesium, antacids, biotin | Take levothyroxine on an empty stomach, 30–60 minutes before breakfast. Leave at least 4 hours before taking iron, calcium or magnesium (including multivitamins containing iron). Note: biotin can interfere with laboratory thyroid blood tests — tell the person taking your blood if you take biotin. |
|
Tetracycline antibiotics (doxycycline, lymecycline, minocycline) |
Iron, zinc, magnesium, calcium | Leave at least 2 to 3 hours between your antibiotic dose and any mineral-containing supplement. |
|
Quinolone antibiotics (ciprofloxacin) |
Iron, zinc, calcium, magnesium | Leave at least 2 hours between doses (and avoid taking ciprofloxacin with dairy or calcium-fortified drinks). |
|
Bisphosphonates (alendronic acid, risedronate, ibandronate) |
Calcium, iron, magnesium, zinc | Take alendronic acid first thing with plain tap water and wait at least 30 minutes before food, drink or supplements (leave at least 2 hours for risedronate, and 6 hours before / 1 hour after for ibandronate). |
| Warfarin & anticoagulants | Vitamin K (K1/K2), omega-3 fish oil, St John's wort | Do not take vitamin K supplements or St John's wort while on warfarin. Vitamin K works against warfarin and alters your INR. Because omega-3 fish oil can also affect bleeding risk, always check with your anticoagulant clinic or pharmacist before starting any supplement. |
| Thiazide diuretics (bendroflumethiazide, indapamide) & digoxin | High-dose Vitamin D & calcium | Combining thiazide water tablets with high-dose vitamin D can increase the risk of raised blood calcium (which also affects digoxin). Check with your GP or pharmacist first. |
| Levodopa (co-careldopa / Sinemet, co-beneldopa / Madopar) & specialist chelation-sensitive medicines (such as dolutegravir, penicillamine, eltrombopag) | Iron, calcium, magnesium, zinc (and high-dose vitamin B6 if taking levodopa without a dopa-decarboxylase inhibitor) | Iron and other divalent minerals can bind to levodopa and certain specialist medicines (such as integrase inhibitors like dolutegravir) in the gut and reduce absorption. Leave at least 2 to 4 hours between doses, and check timing with your pharmacist. |
Read our full interaction guide: Supplements and common medicines: what not to mix →
3. Quick-check table: Which Dr Marks products contain these nutrients?
Because multivitamins contain several minerals and fat-soluble vitamins under one label, use this table to check quickly whether a Dr Marks product contains a nutrient you need to space from your prescription (or avoid on warfarin):
| Nutrient | Present in these Dr Marks products | Not present in |
|---|---|---|
|
Vitamin K2 (MK-7) (Avoid with warfarin) |
Vitamin D3 4,000 IU + K2 (100µg K2), Women's Daily Multivitamin (75µg), Men's Daily Multivitamin (75µg), Over 50s Multivitamin (75µg), Pregnancy Multivitamin (75µg) | Triple B12, Vitamin B Complex, Triple Magnesium, Dual Vitamin C with Zinc, Omega 3 Fish Oil |
|
Iron (Space 4 hrs from levothyroxine; 2–3 hrs from tetracyclines) |
Women's Daily Multivitamin (18mg), Pregnancy Multivitamin (17mg), Men's Daily Multivitamin (9mg), Over 50s Multivitamin (9mg) | Vitamin D3 + K2, Triple B12, Vitamin B Complex, Triple Magnesium, Dual Vitamin C with Zinc, Omega 3 Fish Oil |
|
Zinc (Space 2–3 hrs from tetracyclines/quinolones/bisphosphonates) |
Dual Vitamin C with Zinc (10mg per capsule / 20mg per 2 capsules), Men's (15mg), Over 50s (15mg), Women's (12.5mg), Pregnancy (12mg) | Vitamin D3 + K2, Triple B12, Vitamin B Complex, Triple Magnesium, Omega 3 Fish Oil |
|
Calcium (Space 4 hrs from levothyroxine) |
Dual Vitamin C with Zinc (contains calcium ascorbate, providing ~28.5mg calcium per capsule / ~57mg per 2 capsules) | All 9 other single pouches |
|
Magnesium (Space 2–4 hrs from levothyroxine, tetracyclines, quinolones & bisphosphonates; caution in kidney disease) |
Triple Magnesium (96mg per capsule / 192mg per 2 capsules) | All 9 other single pouches |
|
Biotin (Vitamin B7) (Mention before thyroid/hormone blood tests) |
Vitamin B Complex (200µg), Women's (200µg), Men's (75µg), Over 50s (75µg), Pregnancy (50µg) | Triple B12, Vitamin D3 + K2, Triple Magnesium, Dual Vitamin C with Zinc, Omega 3 Fish Oil |
|
Manganese (Do not combine multiple multivitamins; check with GP in liver disease) |
Men's Daily Multivitamin (3mg), Women's Daily Multivitamin (3mg), Over 50s Multivitamin (3mg), Pregnancy Multivitamin (3mg) | Vitamin D3 + K2, Triple B12, Vitamin B Complex, Triple Magnesium, Dual Vitamin C with Zinc, Omega 3 Fish Oil |
Browse all Dr Marks supplements & full ingredient labels →
References
- British National Formulary (BNF). Interactions: levothyroxine; iron; zinc; magnesium; warfarin; doxycycline; colecalciferol. https://bnf.nice.org.uk/interactions/ (accessed 24 October 2026)
- Medicines and Healthcare products Regulatory Agency (2022). Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. https://www.gov.uk/drug-safety-update/metformin-and-reduced-vitamin-b12-levels-new-advice-for-monitoring-patients-at-risk (accessed 24 October 2026)
- Medicines and Healthcare products Regulatory Agency (2012). Proton pump inhibitors in long-term use: reports of hypomagnesaemia. https://www.gov.uk/drug-safety-update/proton-pump-inhibitors-in-long-term-use-reports-of-hypomagnesaemia (accessed 24 October 2026)
- NHS. Medicines A to Z (Levothyroxine, Warfarin, Omeprazole, Metformin, Alendronic acid, Doxycycline, Ciprofloxacin). https://www.nhs.uk/medicines/ (accessed 24 October 2026)
- Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. https://www.gov.uk/government/publications/great-britain-nutrition-and-health-claims-nhc-register (accessed 24 October 2026)
Food supplements should not replace a varied, balanced diet and healthy lifestyle, and are not intended to diagnose, treat, cure or prevent any disease or medication side effect. If you are pregnant, breastfeeding, taking any prescribed medication or under medical supervision, speak to your GP or pharmacist before taking supplements. Dr Marks is an independent company and is not affiliated with or endorsed by the NHS or the MHRA.
About the authors
Dr Mark Wilshere MBChB MRCGP DCH BSc (Hons), GP and Co-Founder, Dr Marks (medical reviewer) — GMC no. 6144605
Dr Marc Crutchley MBChB MRCGP MSc BSc (Hons), GP and Co-Founder, Dr Marks — GMC no. 6144602
Last reviewed: 24 October 2026 · Our medical review process