Yes, some do. But the risk is not spread evenly across your shelf, and most of what people worry about is the wrong half of the problem. There are two directions of interference: a medicine can lower your nutrient status, which is usually slow and correctable, or a supplement can change how much of a medicine reaches your blood, which can be fast and clinically serious. The second direction is the one worth knowing. Three mechanisms cover almost all of it: minerals binding a drug in the gut, a plant extract speeding up drug breakdown in the liver, and a nutrient doing the opposite job to the drug. Timing solves the first. The other two need a conversation.
The direction that matters, and the direction that mostly does not
Most articles about supplements and medicines describe drugs depleting nutrients: long-term acid suppression and vitamin B12, metformin and B12, and so on. That is real, and we have written about it in which nutrients absorb worse as you get older. It is also, in most cases, the mild direction. A nutrient level falls over months, a blood test picks it up, and the fix is straightforward.
The other direction is different. When a supplement changes the amount of a medicine that reaches your bloodstream, the consequence is not a slowly drifting lab value. It is a thyroid dose that stops working, an antibiotic that never reaches the concentration it needs, a contraceptive with a weaker margin, or an anticoagulant that drifts out of range. That is why this article is written from the drug's point of view rather than the nutrient's.
Mechanism one: minerals bind the drug before it can be absorbed
Calcium, magnesium, iron, zinc and aluminium are divalent or trivalent cations. Several drug molecules have a chemical structure that grabs those ions and forms a complex that the gut wall cannot absorb well. The drug is still in your intestine. It just cannot get out of it. This is the single most common supplement to medicine interaction, and it is entirely about what is in your stomach at the same moment.
The numbers are old and unusually clean. In healthy volunteers given 750 mg of ciprofloxacin, taking an aluminium and magnesium antacid 5 to 10 minutes beforehand left only 15.1 percent of the normal ciprofloxacin exposure. Two hours before, 23.2 percent. Four hours before, 70 percent [5]. A separate crossover study found that ferrous sulfate cut the area under the ciprofloxacin curve from about 14.5 to 5.4 micrograms per hour per millilitre, and a multivitamin with zinc cut it to 11.3. With iron, peak ciprofloxacin concentrations fell below the level needed for 90 percent of strains of organisms normally considered susceptible [6]. In plain terms: the tablet was taken, and the infection was being treated with a fraction of the intended dose.
Levothyroxine is the other classic, and it is more sensitive because the dose is measured in micrograms and the therapeutic window is narrow. In fourteen people with treated hypothyroidism who took 300 mg of ferrous sulfate at the same time as their usual thyroxine for twelve weeks, mean TSH rose from 1.6 to 5.4 mU/L, and nine of them reported more hypothyroid symptoms [1]. In fifty postmenopausal women on levothyroxine who added 600 to 1000 mg of calcium a day, TSH was 3.33 mU/L when the two were taken together against 1.93 and 2.16 mU/L when they were separated, and 18 percent had a clearly elevated TSH against none in the separated settings [3]. A 2017 review lists calcium carbonate, calcium citrate, calcium acetate, iron sulfate, ciprofloxacin, aluminium hydroxide and proton pump inhibitors among the interactions that are established and clinically significant for levothyroxine, and states plainly that these can be avoided by separating the two in time [2].
Magnesium belongs on the same list, though more mildly. A 2025 randomised crossover trial in fifteen healthy adults, the first to test this directly, found that magnesium aspartate reduced levothyroxine exposure by 12 percent and magnesium citrate by 7 percent, the latter not statistically significant. The authors still concluded that people on levothyroxine should take it separately from magnesium-containing products, especially where TSH needs to sit in a narrow range [4].
Mechanism two: the supplement speeds up the liver's disposal system
This one is invisible in the gut and only shows up in the blood. Some plant extracts activate a nuclear receptor called PXR, which increases production of drug-metabolising enzymes, mainly CYP3A4, and of the transport protein P-glycoprotein. The body then clears the drug faster than the dose assumes. St John's wort is the textbook case, and the degree of CYP3A4 induction correlates with the hyperforin content of the specific preparation [9].
The consequences have been documented for two decades. A systematic review of case reports and case series concluded that St John's wort lowers cyclosporine blood levels and, in several cases, led to transplant rejection [10]. In twelve women on a combined oral contraceptive, adding St John's wort shortened the half-life of ethinylestradiol from 23.4 to 12.2 hours and increased norethindrone clearance [8]. A later systematic review found increased breakthrough bleeding in three of four eligible studies and, in one, increased follicular growth and probable ovulation, and concluded that the combination raises concern for reduced contraceptive efficacy [7]. Digoxin, tacrolimus, indinavir, warfarin, alprazolam and simvastatin are all on the affected list [9].
We do not sell St John's wort, and this is the section where a supplement brand has the least incentive to be thorough. That is exactly why it belongs here: the herbal aisle, not the vitamin aisle, is where the serious pharmacokinetic interactions live.
Mechanism three: the nutrient does the opposite job to the drug
Vitamin K antagonists such as warfarin and acenocoumarol work by blocking the recycling of vitamin K. Supply extra vitamin K and you are directly opposing the drug. This is not absorption chemistry; it is the same target from the other side.
Two nuances matter, and they point in opposite directions. For vitamin K from food, a systematic review found the evidence conflicting, with an effect on coagulation detectable mainly at higher intakes above roughly 150 micrograms per day, and concluded that the available evidence does not support telling patients to restrict green vegetables. Keeping intake stable is what matters, not making it small [12]. A randomised trial giving 150 micrograms of oral vitamin K daily found no significant difference in time in therapeutic range against placebo [13]. So the dietary panic was largely misplaced.
Supplemental vitamin K2 is a different question, because the doses are concentrated and MK-7 stays in circulation far longer than the K1 in food. In eighteen anticoagulated volunteers, 45 micrograms of MK-7 a day lowered mean INR and uncarboxylated factor II by roughly 40 percent, and even 10 and 20 micrograms a day were independently judged by two haematologists to cause a clinically relevant lowering of INR in at least 40 and 60 percent of subjects respectively [11]. Those are very small amounts.
Where our own products sit on this list
Applying the above to our own shelf, honestly:
Our vitamin D3 and K2 drops deliver 120 micrograms of K2 as MK-7 per 0.25 ml dose, taken every three days. Against the study above, that is not a marginal amount. Our pack already says to consult your physician before use if you take anticoagulants, and we mean it literally: if you are on a vitamin K antagonist, this is not a product to start and mention at your next appointment. That warning is on the label voluntarily. As the next section explains, nothing in EU law required us to put it there.
Our magnesium provides 251 mg of elemental magnesium in one daily serving of two capsules. Magnesium contributes to normal muscle function and to normal functioning of the nervous system, and none of that changes here. But it is a divalent cation, so if you take levothyroxine, a tetracycline or a fluoroquinolone antibiotic, or a bisphosphonate, separate them in time rather than swallowing them together.
Our zinc is 30 mg of zinc picolinate taken with a meal. Zinc contributes to the normal function of the immune system. Zinc is also a divalent cation, and the multivitamin that impaired ciprofloxacin absorption in the study above contained zinc [6]. Same rule: separate from those antibiotics.
Creatine, our B-complex and our hydrogen tablets are not known participants in the three mechanisms described here, with the caveat that the hydrogen tablets contain magnesium and therefore inherit the mineral timing rule. Our laboratory-only items are not consumables and are outside this discussion entirely.
Why your supplement label will never warn you
This is the structural point most people never hear. Under EU law, a medicine must document its interactions twice. Article 11 of Directive 2001/83/EC requires the summary of product characteristics to contain, at section 4.5, "interaction with other medicinal products and other forms of interactions" [16]. Article 59 requires the package leaflet, the paper in the box written for you rather than for your doctor, to list "forms of interaction with other medicinal products and other forms of interaction (e.g. alcohol, tobacco, foodstuffs) which may affect the action of the medicinal product" [16].
Now the supplement side. Article 6 of Directive 2002/46/EC sets out what a food supplement label must carry: the name "food supplement", the categories of nutrients, the recommended daily portion, a warning not to exceed it, a statement that supplements do not replace a varied diet, and a statement to keep the product away from young children [17]. There is no interaction section, because there is no dossier behind a supplement to write one from.
So the asymmetry is by design, not by neglect. Your medicine's leaflet is legally obliged to tell you about food and drink that affect it. Your supplement's label is not obliged to tell you about medicines. That means the only place the two lists ever meet is in your own head, or in a pharmacist's software.
How often does this actually bite?
In a nationally representative sample of Americans aged 62 to 85, dietary supplement use rose from 51.8 percent in 2005 and 2006 to 63.7 percent in 2010 and 2011, and the proportion at risk of a potential major drug interaction rose from about 8.4 to 15.1 percent, with most of those interacting regimens involving medications and supplements whose use was increasing [14]. That is US data on older adults, so do not read it as a European rate. It does establish the shape of the problem: concurrent use is now the norm, not the exception.
And the information often never reaches the clinician. A systematic review of communication about complementary medicine use found that between 20 and 77 percent of patients did not disclose it, with the leading reasons being that the doctor never asked and that patients assumed it was irrelevant to their conventional care [15].
The rule that solves most of it
For mechanism one, which covers most real cases, the fix is boring and effective: put time between the two. The antacid data show the pattern clearly, with ciprofloxacin exposure recovering from 15 percent to 70 percent as the gap widened from minutes to four hours [5]. The levothyroxine review reaches the same conclusion, that separating administration avoids the interaction [2].
A workable default is to take levothyroxine on its own on an empty stomach and to keep mineral supplements at least four hours away from it, and to keep minerals at least two hours after or six hours before a tetracycline or fluoroquinolone antibiotic. Follow the exact interval on your medicine's leaflet where it gives one, because that interval was set for that specific drug. If you want the general logic of when to take what, we covered it in should you take supplements with food or on an empty stomach.
Mechanisms two and three cannot be fixed by timing. A liver enzyme that has been induced stays induced for days, and vitamin K opposes an anticoagulant no matter when you swallow it. Those two need a decision, not a schedule.
The sixty second version for the pharmacy counter
Bring the actual packs or photograph the labels, because "a magnesium" and "a multivitamin" are not enough information to check anything. Say what you take, at what dose, and when. Ask one question: does anything here need to be separated in time from my prescription, or avoided while I am on it? A pharmacist can answer that from an interaction database in under a minute, and it is free.
Three situations deserve that conversation before you start anything, rather than after: you take a vitamin K antagonist, you take levothyroxine or another narrow therapeutic index drug, or you take an immunosuppressant after a transplant. In those three cases, the cost of guessing is not a wasted capsule.
We are a supplement company, not a clinic. Nothing here is medical advice, and none of it replaces your prescriber. If you want to think about what belongs in your routine in the first place, our piece on how to build a supplement stack that actually makes sense is the better starting point.
Frequently asked questions
Can I take magnesium if I am on levothyroxine?
Generally yes, but not at the same moment. The only trial to test this directly found magnesium aspartate reduced levothyroxine exposure by 12 percent and magnesium citrate by 7 percent, and the authors advised taking levothyroxine separately from magnesium-containing products, particularly if your TSH needs to stay in a narrow range [4]. Take the levothyroxine first thing on an empty stomach and the magnesium later in the day, and let your doctor know so your next TSH check is interpreted correctly.
Do I have to stop eating spinach and broccoli on warfarin?
Current evidence does not support restricting vitamin K rich foods. A systematic review found the evidence conflicting, with effects mainly at intakes above roughly 150 micrograms a day, and concluded that keeping your habits stable matters more than keeping them low [12]. A concentrated K2 supplement is a different matter, because small daily amounts of MK-7 measurably lowered INR in anticoagulated volunteers [11].
How long should I leave between a mineral supplement and an antibiotic?
Follow the interval printed in your antibiotic's package leaflet, since that is drug-specific and legally required to be there. As an illustration of why the gap matters, ciprofloxacin exposure was 15.1 percent of normal when an antacid was taken 5 to 10 minutes before, and 70 percent when it was taken four hours before [5].
Are herbal supplements riskier than vitamins and minerals?
For this particular problem, generally yes. Mineral interactions are mostly about timing and are reversible the next day. Plant extracts that induce CYP3A4 and P-glycoprotein, St John's wort being the best documented, change drug clearance for as long as you keep taking them and have caused transplant rejection and contraceptive failure in published cases [7][9][10].
My supplement label says nothing about my medicine. Does that mean it is safe?
No. It means EU law does not ask for that information. Article 6 of Directive 2002/46/EC lists what a supplement label must state, and interactions are not on that list, while Articles 11 and 59 of Directive 2001/83/EC require exactly that information for medicines [16][17]. Absence of a warning on a supplement is absence of a legal requirement, not evidence of safety.
Should I just stop my supplements while I am on a course of antibiotics?
You usually do not need to. For the mineral interaction, separating the doses within the day is enough, and a short antibiotic course is not long enough for a pause to change your nutrient status either way. The decision worth checking with a pharmacist is not whether to pause but whether your specific antibiotic is one of the chelation-sensitive classes.
The bottom line
Supplements do interact with medications, but not randomly and not everywhere. Minerals bind certain antibiotics and levothyroxine in the gut, which timing fixes. Some plant extracts, above all St John's wort, speed up the liver's clearance of drugs, which timing does not fix. Vitamin K opposes vitamin K antagonists directly, and concentrated MK-7 does so at surprisingly small doses. Everything else on a normal shelf is far less eventful than the internet suggests.
The uncomfortable part is structural: the medicine in your cupboard is legally required to disclose its interactions, and the supplement next to it is not. Until that changes, the check has to happen somewhere else, which in practice means a two minute conversation with a pharmacist who can see both lists at once. That is a small price for keeping a prescription working as intended.
Sources
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- Morini E, Catalano A, Lasco A, Morabito N, Benvenga S. L-thyroxine malabsorption due to calcium carbonate impairs blood pressure, total cholesterolemia, and fasting glycemia. Endocrine, 2019. PMID 30368654.
- Attinger MC, von Felten S, Rodrigues CL, Krützfeldt J, Risch L, Bonzon J. Randomized crossover trial on drug-drug interactions of levothyroxine with magnesium citrate and magnesium aspartate (the ThyroMag trial). Clinical and Translational Science, 2025. PMID 41221788.
- Nix DE, Watson WA, Lener ME, et al. Effects of aluminum and magnesium antacids and ranitidine on the absorption of ciprofloxacin. Clinical Pharmacology and Therapeutics, 1989. PMID 2598571.
- Polk RE, Healy DP, Sahai J, Drwal L, Racht E. Effect of ferrous sulfate and multivitamins with zinc on absorption of ciprofloxacin in normal volunteers. Antimicrobial Agents and Chemotherapy, 1989. PMID 2610494.
- Berry-Bibee EN, Kim MJ, Tepper NK, Riley HE, Curtis KM. Co-administration of St. John's wort and hormonal contraceptives: a systematic review. Contraception, 2016. PMID 27444983.
- Hall SD, Wang Z, Huang SM, et al. The interaction between St John's wort and an oral contraceptive. Clinical Pharmacology and Therapeutics, 2003. PMID 14663455.
- Nicolussi S, Drewe J, Butterweck V, Meyer zu Schwabedissen HE. Clinical relevance of St. John's wort drug interactions revisited. British Journal of Pharmacology, 2020. PMID 31742659.
- Ernst E. St John's wort supplements endanger the success of organ transplantation. Archives of Surgery, 2002. PMID 11888457.
- Theuwissen E, Teunissen KJ, Spronk HM, et al. Effect of low-dose supplements of menaquinone-7 (vitamin K2) on the stability of oral anticoagulant treatment: dose-response relationship in healthy volunteers. Journal of Thrombosis and Haemostasis, 2013. PMID 23530987.
- Violi F, Lip GY, Pignatelli P, Pastori D. Interaction between dietary vitamin K intake and anticoagulation by vitamin K antagonists: is it really true? A systematic review. Medicine (Baltimore), 2016. PMID 26962786.
- Boonyawat K, Wang L, Lazo-Langner A, et al. The effect of low-dose oral vitamin K supplementation on INR stability in patients receiving warfarin: a randomised trial. Thrombosis and Haemostasis, 2016. PMID 27346552.
- Qato DM, Wilder J, Schumm LP, Gillet V, Alexander GC. Changes in prescription and over-the-counter medication and dietary supplement use among older adults in the United States, 2005 vs 2011. JAMA Internal Medicine, 2016. PMID 26998708.
- Davis EL, Oh B, Butow PN, Mullan BA, Clarke S. Cancer patient disclosure and patient-doctor communication of complementary and alternative medicine use: a systematic review. The Oncologist, 2012. PMID 22933591.
- Directive 2001/83/EC on the Community code relating to medicinal products for human use, Articles 11 and 59. Article 11 and Article 59. EUR-Lex serves an empty response to automated requests, so these links point to the legislation.gov.uk mirror of the EU instrument.
- Directive 2002/46/EC on food supplements, Article 6 (labelling particulars). Same mirror, same limitation as above.
