Supplement and drug interactions
Interactions with medications described in the NIH Office of Dietary Supplements (ODS) Health Professional fact sheets. Each drug links to its page in the ClinDose drug library.
Vitamin K
ODS fact sheet, updated Oct 22, 2026Warfarin (Coumadin) and similar anticoagulants
Vitamin K can have a serious and potentially dangerous interaction with anticoagulants such as warfarin (Coumadin) as well as phenprocoumon, acenocoumarol, and tioclomarol, which are commonly used in some European countries. People who are taking warfarin and similar anticoagulants need to maintain a consistent intake of vitamin K from food and supplements because sudden changes in vitamin K intakes can increase or decrease the anticoagulant effect.
Antibiotics
Antibiotics can destroy vitamin K-producing bacteria in the gut, potentially decreasing vitamin K status. This effect might be more pronounced with cephalosporin antibiotics, such as cefoperazone (Cefobid), because these antibiotics might also inhibit the action of vitamin K in the body.
Bile acid sequestrants
Bile acid sequestrants, such as cholestyramine (Questran) and colestipol (Colestid), are used to reduce cholesterol levels by preventing reabsorption of bile acids. They can also reduce the absorption of vitamin K and other fat-soluble vitamins, although the clinical significance of this effect is not clear.
Orlistat
Orlistat is a weight-loss drug that is available as both an over-the-counter (Alli) and prescription (Xenical) medication. It reduces the body's absorption of dietary fat and in doing so, it can also reduce the absorption of fat-soluble vitamins, such as vitamin K.
Vitamin E
ODS fact sheet, updated Oct 19, 2026Anticoagulant and antiplatelet medications
Vitamin E can inhibit platelet aggregation and antagonize vitamin K-dependent clotting factors. As a result, taking large doses of vitamin E with anticoagulant or antiplatelet medications, such as warfarin (Coumadin), can increase the risk of bleeding, especially in conjunction with low vitamin K intake.
Simvastatin and niacin
Some people take vitamin E supplements with other antioxidants, such as vitamin C, selenium, and beta-carotene. This collection of antioxidant ingredients blunted the rise in high-density lipoprotein (HDL) cholesterol levels among people who were treated with a combination of simvastatin (brand name Zocor) and niacin.
Chemotherapy and radiotherapy
Oncologists generally advise against the use of antioxidant supplements during cancer chemotherapy or radiotherapy because they might reduce the effectiveness of these therapies by inhibiting cellular oxidative damage in cancerous cells. Although a systematic review of randomized controlled trials has called this concern into question, further research is needed to evaluate the potential risks and benefits of concurrent antioxidant supplementation with conventional therapies…
No matching drug in the library.
Vitamin D
ODS fact sheet, updated Jun 27, 2025Orlistat
The weight-loss drug orlistat (Xenical and alli), together with a reduced-fat diet, can reduce the absorption of vitamin D from food and supplements, leading to lower 25(OH)D levels.
Statins
Statin medications reduce cholesterol synthesis. Because endogenous vitamin D is derived from cholesterol, statins may also reduce vitamin D synthesis.
Steroids
Corticosteroid medications, such as prednisone (Deltasone, Rayos, and Sterapred), are often prescribed to reduce inflammation. These medications can reduce calcium absorption and impair vitamin D metabolism.
Thiazide diuretics
Thiazide diuretics (e.g., Hygroton, Lozol, and Microzide) decrease urinary calcium excretion. The combination of these diuretics with vitamin D supplements (which increase intestinal calcium absorption) might lead to hypercalcemia, especially among older adults and individuals with compromised renal function or hyperparathyroidism.
Vitamin A
ODS fact sheet, updated Mar 10, 2025Retinoids
Several synthetic retinoids derived from vitamin A are used orally as prescription medicines. Retinoids can increase the risk of hypervitaminosis A when taken in combination with vitamin A supplements.
Vitamin C
ODS fact sheet, updated Jul 31, 2025Chemotherapy and radiation
The safety and efficacy of using vitamin C and other antioxidants during cancer treatment is controversial. Individuals who are undergoing chemotherapy or radiation should consult with their oncologist prior to taking vitamin C or other antioxidant supplements, especially in high doses.
No matching drug in the library.
3-Hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins)
Vitamin C, in combination with other antioxidants, may attenuate the increase in high-density lipoprotein levels that result from taking combination niacin/simvastatin (Zocor) therapy. Health care providers should monitor lipid levels in individuals who are taking both statins and antioxidant supplements.
Vitamin B6
ODS fact sheet, updated Jun 16, 2023Cycloserine
Cycloserine (Seromycin) is a broad-spectrum antibiotic used to treat tuberculosis. In combination with pyridoxal phosphate, cycloserine increases urinary excretion of pyridoxine.
Antiepileptic medications
Some antiepileptic drugs, including valproic acid (Depakene, Stavzor), carbamazepine (Carbatrol, Epitol, Tegretol, and others), and phenytoin (Dilantin) increase the catabolism rate of vitamin B6 vitamers, resulting in low plasma PLP concentrations and hyperhomocysteinemia.
Theophylline
Theophylline (Aquaphyllin, Elixophyllin, Theolair, Truxophyllin, and many others) can prevent or treat shortness of breath, wheezing, and other breathing problems caused by asthma, chronic bronchitis, emphysema, and other lung diseases. Patients treated with theophylline often have low plasma PLP concentrations, which could contribute to the neurological and central nervous system side effects associated with theophylline, including seizures.
Vitamin B12
ODS fact sheet, updated Jul 2, 2025Gastric acid inhibitors
Gastric acid inhibitors include proton pump inhibitors, such as omeprazole (Prilosec) and lansoprazole (Prevacid), and histamine 2-receptor antagonists, such as cimetidine (Tagamet) and ranitidine (Zantac). They can interfere with vitamin B12 absorption from food by slowing the release of gastric acid into the stomach and thereby lead to vitamin B12 deficiency.
Metformin
Metformin, an antihyperglycemic agent used as first-line treatment for prediabetes and diabetes, might reduce the absorption of vitamin B12 and significantly reduce serum vitamin B12 concentrations.
Folate (folic acid)
ODS fact sheet, updated Nov 30, 2022Methotrexate
Methotrexate (Rheumatrex, Trexall), used to treat cancer and autoimmune diseases, is a folate antagonist. Patients taking methotrexate for cancer should consult their oncologist before taking folate supplements because the supplements could interfere with methotrexate’s anticancer effects.
Antiepileptic medications
Antiepileptic medications, such as phenytoin (Dilantin), carbamazepine (Carbatrol, Tegretol, Equetro, Epitol), and valproate (Depacon), are used to treat epilepsy, psychiatric diseases, and other medical conditions. These medications can reduce serum folate levels.
Sulfasalazine
Sulfasalazine (Azulfidine) is used primarily to treat ulcerative colitis. It inhibits the intestinal absorption of folate and can cause folate deficiency.
Isoniazid and pyrazinamide
Isoniazid and pyrazinamide (together in Rifater), used to treat tuberculosis, are structural analogs of niacin and interrupt the production of niacin from tryptophan by competing with a vitamin B6-dependent enzyme required for this process. In addition, isoniazid can interfere with niacin's conversion to NAD.
Antidiabetes medications
Large doses of nicotinic acid can raise blood glucose levels by causing or aggravating insulin resistance and increasing hepatic production of glucose. 5 g/day or more are most likely to increase blood glucose levels in individuals with or without diabetes.
Furosemide
Furosemide (Lasix) is a loop diuretic used to treat edema and hypertension by increasing urinary output. Research has linked the use of furosemide to decreases in thiamin concentrations, possibly to deficient levels, as a result of urinary thiamin loss.
Chemotherapy with fluorouracil
Fluorouracil (also known as 5-fluorouracil; Adrucil) is a chemotherapy drug that is commonly used to treat colorectal and other solid cancers. The published literature includes several cases of beriberi or Wernicke’s encephalopathy resulting from treatment with this drug, possibly because the drug might increase thiamin metabolism and block the formation of TDP, the active form of thiamin.
Dolutegravir
Dolutegravir (Dovato, Tivicay) is an HIV integrase inhibitor used in adults and children. Concomitant use of calcium supplements and dolutegravir can reduce blood levels of dolutegravir substantially, apparently through chelation.
Levothyroxine
Calcium carbonate supplements can interfere with the absorption of levothyroxine (Synthroid, Levoxyl, and others), a thyroid hormone used to treat hypothyroidism and thyroid cancer. The FDA-approved label for this medication instructs patients who are taking calcium carbonate supplements to avoid taking levothyroxine within 4 hours of taking the supplement.
Lithium
Long-term use of lithium (Eskalith, Lithobid), a treatment for bipolar disorder, can lead to hypercalcemia, and use of both lithium and calcium supplements could increase this risk.
Quinolone antibiotics
Simultaneous use of calcium supplements and quinolone antibiotics—such as ciprofloxacin (Cipro), gemifloxacin (Factive), and moxifloxacin (Avelox)—can reduce the absorption of quinolones. Taking the antibiotic 2 hours before or 2 hours after calcium supplements prevents this effect.
Magnesium
ODS fact sheet, updated Jan 6, 2026Bisphosphonates
Magnesium-rich supplements or medications can decrease the absorption of oral bisphosphonates, such as alendronate (Fosamax), that are used to treat osteoporosis. Taking magnesium-rich supplements or medications at least 2 hours before or after oral bisphosphonates can minimize this interaction.
Antibiotics
Magnesium can form insoluble complexes with tetracyclines, such as demeclocycline (Declomycin) and doxycycline (Vibramycin), and with quinolone antibiotics, such as ciprofloxacin (Cipro) and levofloxacin (Levaquin). These antibiotics should be taken at least 2 hours before or 4 to 6 hours after a magnesium-containing supplement.
Diuretics
Chronic treatment with loop diuretics, such as furosemide (Lasix) and bumetanide (Bumex), and thiazide diuretics, such as hydrochlorothiazide (Aquazide H) and ethacrynic acid (Edecrin), can increase the loss of magnesium in urine and lead to magnesium depletion. In contrast, potassium-sparing diuretics, such as amiloride (Midamor) and spironolactone (Aldactone), reduce magnesium excretion.
Proton pump inhibitors
Prescription proton pump inhibitor (PPI) drugs, such as esomeprazole magnesium (Nexium) and lansoprazole (Prevacid), can cause hypomagnesemia when taken for prolonged periods (typically more than a year). In the cases that FDA reviewed, magnesium supplements often raised the low serum magnesium levels caused by PPIs.
Levodopa
Some evidence indicates that in healthy people, iron supplements reduce the absorption of levodopa (found in Sinemet and Stalevo), used to treat Parkinson’s disease and restless leg syndrome, possibly through chelation. In the United States, the labels for levodopa warn that iron-containing dietary supplements might reduce the amount of levodopa available to the body and, thus, diminish its clinical effectiveness.
Levothyroxine
Levothyroxine (Levothroid, Levoxyl, Synthroid, Tirosint, and Unithroid) is used to treat hypothyroidism, goiter, and thyroid cancer. The labels for some of these products warn that iron supplements can reduce the absorption of levothyroxine tablets and advise against administering levothyroxine within 4 hours of iron supplements.
Proton pump inhibitors
Gastric acid plays an important role in the absorption of nonheme iron from the diet. Because proton pump inhibitors, such as lansoprazole (Prevacid) and omeprazole (Prilosec), reduce the acidity of stomach contents, they can reduce iron absorption.
Antibiotics
Both quinolone antibiotics (such as Cipro) and tetracycline antibiotics (such as Achromycin and Sumycin) might interact with zinc in the gastrointestinal tract, which could inhibit the absorption of both zinc and the antibiotic if they are taken at the same time. Taking the antibiotic at least 2 hours before or 4 to 6 hours after the zinc supplement minimizes this interaction.
Penicillamine
Zinc can reduce the absorption and action of penicillamine, a drug used to treat rheumatoid arthritis and Wilson disease. To minimize this interaction, people should take zinc supplements and penicillamine at least 1 hour apart.
Diuretics
Thiazide diuretics, such as chlorthalidone (e.g., Hygroton, Thalitone) and hydrochlorothiazide (e.g., Esidrix, HydroDIURIL), increase zinc excretion in the urine. This increased excretion, in turn, decreases serum zinc concentrations.
Potassium
ODS fact sheet, updated Jun 2, 2022Angiotensin converting enzyme inhibitors and angiotensin receptor blockers
ACE inhibitors, such as benazepril (Lotensin), and angiotensin receptor blockers (ARBs) such as losartan (Cozaar), are used to treat hypertension and heart failure, slow progression of kidney disease in patients with chronic kidney disease and type 2 diabetes, and decrease morbidity and mortality after myocardial infarction. These medications reduce urinary potassium excretion, which can lead to hyperkalemia.
Potassium-sparing diuretics
Potassium-sparing diuretics, such as amiloride (Midamor) and spironolactone (Aldactone), reduce the excretion of potassium in the urine and can cause hyperkalemia. Experts recommend monitoring potassium status in people taking these medications, especially if they have impaired kidney function or other risk factors for hyperkalemia.
Loop and thiazide diuretics
Treatment with loop diuretics, such as furosemide (Lasix) and bumetanide (Bumex), and thiazide diuretics, such as chlorothiazide (Diuril) and metolazone (Zaroxolyn), increases urinary potassium excretion and can lead to hypokalemia. Experts recommend monitoring potassium status in people taking these medications and initiating potassium supplementation if warranted.
Phosphorus
ODS fact sheet, updated May 4, 2023Antacids
Antacids that contain aluminum hydroxide, such as Maalox HRF and Rulox, bind phosphorus in the intestines, and their chronic use for 3 months or longer can therefore lead to hypophosphatemia. Antacids containing calcium carbonate (Rolaids, Tums, Maalox) also decrease instestinal absorption of dietary phosphorus.
Laxatives
Some laxatives, such as Fleet Prep Kit #1, contain sodium phosphate, and ingesting these products can increase serum phosphate levels. After 13 reports of deaths associated with taking one dose that was higher than recommended on the label of a laxative containing sodium phosphate, FDA issued a warning that these products are potentially dangerous if more than recommended doses are taken, especially in people with kidney disease, heart disease, or dehydration.
No matching drug in the library.
Antithyroid medications
Antithyroid medications, such as methimazole (Tapazole), are used to treat hyperthyroidism. Taking high doses of iodine with antithyroid medications can have an additive effect and could cause hypothyroidism.
Angiotensin-converting enzyme inhibitors
Angiotensin-converting enzyme (ACE) inhibitors, such as benazepril (Lotensin), lisinopril (Prinivil and Zestril), and fosinopril (Monopril), are used primarily to treat high blood pressure. Taking potassium iodide with ACE inhibitors can increase the risk of hyperkalemia (elevated blood levels of potassium).
Potassium-sparing diuretics
Taking potassium iodide with potassium-sparing diuretics, such as spironolactone (Aldactone) and amiloride (Midamor), can increase the risk of hyperkalemia.
Cisplatin
Cisplatin, an inorganic platinum chemotherapy agent, is used to treat ovarian, bladder, lung, and other cancers. Cisplatin can reduce selenium levels in hair and serum but whether these reductions have a clinically significant impact is not known.
Insulin
Chromium might increase insulin sensitivity. Taking chromium concomitantly with insulin could increase the risk of hypoglycemia.
No matching drug in the library.
Metformin and other antidiabetes medications
The results from some studies indicate that chromium supplementation might lower blood glucose levels. Therefore, chromium supplements might have an additive effect with metformin or other antidiabetes medications and thus might increase the risk of hypoglycemia.
Levothyroxine
A small study found that taking chromium picolinate supplements at the same time as levothyroxine (used to treat hypothyroidism) decreases levothyroxine absorption over 6 hours.
Omega-3 fatty acids (fish oil)
ODS fact sheet, updated Aug 22, 2025Warfarin (Coumadin) and similar anticoagulants
Fish oil can have antiplatelet effects at high doses, although it appears to be less potent than aspirin. The authors of a 2014 review concluded that omega-3s do not affect the risk of clinically significant bleeding, and the FDA-approved package inserts for omega-3 pharmaceuticals state that studies with omega-3s have not produced "clinically significant bleeding episodes".
Ashwagandha
ODS fact sheet, updated May 2, 2025Thyroid hormone medications
Ashwagandha might interact with thyroid hormone medications, a suggestion based on case reports of thyrotoxicosis.
Other medications
Ashwagandha might also interact with antidiabetes medications, antihypertensives, immunosuppressants, and sedatives.
Source: NIH Office of Dietary Supplements fact sheets (public domain), checked Nov 4, 2026. Drug classes named in a fact sheet (for example, proton pump inhibitors) are linked to each member drug. This list covers the ODS fact sheets only; check the drug label and an interaction checker for other supplements.