Interactions between dietary supplements (NEMs) and medications

June 26, 2024
Interaktionen von Nahrungsergänzungsmitteln (NEMs) und Medikamenten
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You may be wondering how dietary supplements (DSSs) can affect your medications, especially if you are vegan and use DSSs to supplement your diet. In this article, we will take a closer look at how DSSs and medications can interact with each other and what effect this can have on your health. 

What are dietary supplements?  

Dietary supplements are products that provide vitamins, minerals, trace elements, amino acids, enzymes or other substances in concentrated form to support your diet. They can be available in different forms such as tablets , capsules, powders or liquids. 

The challenge: interactions between Supplements and medications 

The problem is that there can be potential interactions between Supplements and drugs. There are three main types of interactions between substances (xenobiotics): 

  1. Direct chemical-chemical interactions: Substances react with each other in the body and can influence each other. 
  1. Pharmacodynamic interactions: Substances can alter the effect of other substances without affecting their concentration in the blood. This can have additive or antagonistic effects. 
  1. Pharmacokinetic interactions: Substances can affect the absorption, distribution, metabolism or excretion of other substances, altering their concentration in the body. 

Let’s take a closer look at the most common possible interactions: 

  • Vitamin K and blood thinners (e.g. warfarin): Vitamin K is crucial for the formation of blood clotting factors. However, warfarin blocks the action of vitamin K to thin the blood. An excess of vitamin K from supplements can reduce the effectiveness of warfarin by eliminating its inhibition. 
  • Iron and thyroid medications (e.g. levothyroxine): Iron can interfere with the absorption of thyroid hormones by affecting their binding to receptors. This can lead to reduced effectiveness of thyroid medications. 
  • Calcium and antibiotics (e.g. tetracyclines): Calcium can reduce the absorption of tetracycline antibiotics by binding to the drugs and causing the formation of insoluble complexes, thereby reducing the availability of the antibiotics in the body. 
  • Grapefruit and certain medications: Grapefruit contains compounds that inhibit the CYP3A4 enzyme in the liver. This enzyme is responsible for breaking down many medications. When it is inhibited, the medications can stay in the body longer and reach higher concentrations, which can lead to increased effects or side effects. 
  • Folic acid and methotrexate (for rheumatoid arthritis and cancer): Methotrexate is a folic acid antagonist, and taking folic acid supplements may reduce the effect of methotrexate, which may make treatment less effective. 
  • Magnesium and antibiotics: Magnesium can interfere with the absorption of antibiotics by interfering with their absorption in the intestine. This can result in fewer antibiotics reaching the bloodstream and weakening their effect. 
  • Vitamin D and steroids: High doses of vitamin D can enhance the anti-inflammatory effects of steroids by interfering with the immune response and inhibiting inflammation. 
  • Potassium and ACE inhibitors (e.g. enalapril): The combination of potassium supplements with ACE inhibitors can lead to increased potassium levels in the blood. ACE inhibitors can reduce the excretion of potassium, while potassium supplements increase potassium absorption, which leads to accumulation and increases the risk of cardiac arrhythmias. 
  • Omega-3 fatty acids and blood-thinning medications (eg, warfarin): Omega-3 fatty acids can inhibit platelet aggregation and increase the tendency to bleed, especially when taken together with blood-thinning medications such as aspirin or warfarin. 
  • Vitamin E and blood thinners (eg, aspirin): Vitamin E can affect blood clotting and increase the effect of blood thinners such as aspirin. This increases the risk of bleeding. 
  • Calcium and bisphosphonates (for osteoporosis): Taking calcium supplements can reduce the absorption of bisphosphonates used to treat osteoporosis. Calcium can inhibit the absorption of bisphosphonates in the intestine. 
  • Vitamin C and chemotherapy: Some studies suggest that high doses of vitamin C supplements may reduce the effectiveness of some chemotherapy drugs. This may be because vitamin C acts as an antioxidant and reduces the oxidative effects of chemotherapy. 
  • Zinc and antibiotics (e.g., tetracyclines): Zinc supplements can reduce the absorption of antibiotics such as tetracyclines because they react with each other in the intestinal wall and form poorly soluble complexes. 


Tips for the safe use of nutritional supplements and medications  

Here are some tips to ensure you can safely combine Supplements and medications: 

Talk to your doctor: Inform your doctor about all the nutritional supplements you plan to take and discuss your current medications. 

Ask your pharmacist: Ask your pharmacist about possible interactions and the best time to take your Supplements in relation to your medications. 

Read the labels & package inserts: Make sure to read the labels & package inserts of your Supplements carefully and stick to the recommended dosage. 

Monitor your health: Be aware of any changes in your well-being while taking supplements and medications, and report any adverse reactions or symptoms to your doctor immediately. 

Regular check-ups: Schedule regular health checks to ensure your diet and medications are appropriate and there are no deficiencies or excesses. 

Remember that interactions between supplements and medications can depend on many factors, including the specific medications and supplements, dosages, and individual health conditions. Therefore, it is advisable to seek professional advice to ensure you are taking your medications and supplements safely and effectively. 



  1. Institute of Medicine (US) and National Research Council (US) Committee on the Framework for Evaluating the Safety of Dietary Supplements. Dietary supplements: a framework for evaluating safety. Washington (DC): National Academies Press (US); 2005. 8, Interactions. Available from: 
  2. Prescott, Jeffery David et al. “Medications and Micronutrients: Identifying Clinically Relevant Interactions and Addressing Nutritional Needs.” The Journal of pharmacy technology : jPT : official publication of the Association of Pharmacy Technicians vol. 34,5 (2018): 216-230. doi:10.1177/8755122518780742 
  3. Péter, S., Navis, G., de Borst, MH, von Schacky, C., van Orten-Luiten, ACB, Zhernakova, A., Witkamp, ​​RF, Janse, A., Weber, P., Bakker, SJL, & Eggersdorfer, M. (2017). Public health relevance of drug-nutrition interactions. European journal of nutrition, 56(Suppl 2), 23–36. 
  4. Amit Sood, Richa Sood, Francis J. Brinker, Ravneet Mann, Laura L. Loehrer, Dietlind L. Wahner-Roedler, Potential for Interactions Between Dietary Supplements and Prescription Medications, The American Journal of Medicine, Volume 121, Issue 3, 2008, Pages 207-211, ISSN 0002-9343, . 
  5. Donaldson, M. (2014). Impacts and Interrelationships Between Medications, Nutrition, Dietary Supplements, and Oral Health. In: Touger-Decker, R., Mobley, C., Epstein, J. (eds) Nutrition and Oral Medicine. Nutrition and Health. New York: Harvard Business Review, New York, NY. 
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