absorption
75 interactions related to absorption
levothyroxine + magnesium
Taking magnesium too close to levothyroxine can modestly reduce how much of the thyroid medicine is absorbed, because magnesium can bind levothyroxine in the gut.
oat fiber + red yeast rice
Soluble, viscous fibers like oat fiber can bind and slow the absorption of the statin-like compound (monacolin K) in red yeast rice when the two are taken together. Because monacolin K is chemically identical to prescription lovastatin, the documented effect of pectin and oat bran on lovastatin absorption applies directly: co-ingested soluble fiber can reduce how much of the active statin reaches the bloodstream, blunting red yeast rice's cholesterol-lowering effect. The effect is about lost benefit rather than a safety hazard, and it is reversible when the two are separated in time.
antibiotics + calcium
Calcium can bind to certain antibiotics (tetracyclines and fluoroquinolones) in the gut and reduce how much of the drug is absorbed.
levothyroxine + iron
When taken at the same time, iron can reduce how much levothyroxine your body absorbs by forming a poorly soluble complex in the gut, which can blunt the effect of your thyroid medication and raise TSH.
omega-3 + vitamin d
Fat from omega-3 supports absorption of the fat-soluble vitamin D
vitamin d + magnesium
Magnesium helps activate and support the function of vitamin D; low magnesium can reduce the effectiveness of vitamin D supplementation. This is a beneficial nutrient synergy rather than a harmful interaction.
St. John's Wort + SSRI
St. John's Wort is pharmacologically active, not a harmless herb, and it interacts with SSRIs in two overlapping and hard-to-predict ways. The result is a combination most clinicians prefer to avoid rather than manage.
antibiotics + probiotics
Taken at the same moment, an antibiotic can kill bacterial probiotic organisms before they reach the gut, lowering the probiotic's benefit. Spacing the doses apart fixes it.
doxycycline + iron
Iron forms an insoluble chelate complex with doxycycline in the gut, sharply reducing absorption of the antibiotic. In controlled human studies, ferrous sulfate taken together with doxycycline cut serum antibiotic levels substantially, which can undermine treatment.
curcumin + piperine
Piperine (black pepper extract) substantially increases how much curcumin your body absorbs.
atenolol + calcium
Calcium supplements and calcium-based antacids taken at the same time as atenolol bind it in the gut and reduce how much of the drug is absorbed, blunting its blood-pressure and heart-rate effects. Separating the two doses by several hours preserves atenolol's effect. Calcium from ordinary meals is generally not a concern.
doxycycline + magnesium
Magnesium ions can bind doxycycline in the gastrointestinal tract, forming a poorly absorbed complex that reduces how much antibiotic reaches the bloodstream. Magnesium-containing supplements, antacids, and laxatives can meaningfully lower doxycycline absorption if taken at the same time.
levothyroxine + calcium
Calcium can reduce levothyroxine absorption when the two are taken close together
metformin + vitamin b12
Long-term metformin use can reduce vitamin B12 absorption, sometimes enough to cause deficiency.
carbamazepine + biotin
Carbamazepine gradually lowers biotin (vitamin B7) status by reducing intestinal absorption, increasing urinary loss, and accelerating breakdown of the vitamin. The effect is biomarker-level and well documented over decades; frank deficiency and serious adult harm are uncommon.
doxycycline + calcium
Calcium binds doxycycline in the gut, forming a complex the body cannot fully absorb. Taking doxycycline together with calcium supplements, calcium-based antacids, or large dairy servings can lower how much antibiotic reaches the bloodstream, though doxycycline binds calcium less than older tetracyclines.
vitamin c + iron
Vitamin c enhances absorption of non-heme iron from supplements and plant foods, a beneficial nutrient synergy, though the real-world benefit across a full diet is usually modest.
calcium + iron
Calcium can reduce the absorption of iron when the two are taken together, with the effect most pronounced for non-heme iron from supplements and plant foods.
iron + zinc
High-dose iron and zinc supplements can compete for absorption in the small intestine when taken together, especially in solution on an empty stomach, potentially reducing the effectiveness of one or both minerals. The competition is minimal when the minerals are taken with food or hours apart, or at ordinary dietary amounts.
omeprazole + magnesium
Long-term omeprazole use (typically more than a year, occasionally sooner) can lower body magnesium, likely by impairing active intestinal magnesium transport through the TRPM6/TRPM7 channels. The FDA issued a formal Drug Safety Communication in 2011 warning that prescription proton pump inhibitors can cause hypomagnesemia, with serious cases involving abnormal heart rhythm, muscle spasm (tetany), and seizures.
omeprazole + calcium
Omeprazole strongly suppresses stomach acid, and calcium carbonate (the most common supplemental form) needs that acid to dissolve and be absorbed efficiently, especially on an empty stomach. Calcium citrate absorbs well regardless of stomach acid. Long-term proton pump inhibitor use is also associated with a modestly increased risk of hip, wrist, and spine fractures, which prompted an FDA labeling change.
famotidine + vitamin b12
Long-term daily famotidine lowers stomach acid, which can modestly reduce absorption of vitamin B12 bound to food. Supplemental and fortified-food B12 are unaffected because they do not require stomach acid for absorption.
pantoprazole + magnesium
Pantoprazole, like all proton pump inhibitors (PPIs), is associated with low magnesium (hypomagnesemia) after long-term use, likely by impairing active intestinal magnesium transport. The FDA included pantoprazole in its 2011 Drug Safety Communication on PPI-induced hypomagnesemia, which in severe cases can cause arrhythmia, tetany, and seizures.
esomeprazole + vitamin b12
Esomeprazole and other PPIs suppress stomach acid, which can reduce absorption of food-bound vitamin B12 over long-term use; crystalline B12 in supplements is unaffected.
risedronate + calcium
Calcium and other divalent cations bind risedronate in the gut and form insoluble complexes, blocking absorption of a drug whose oral bioavailability is already very low. Taking them together can reduce the absorbed amount to subtherapeutic levels and quietly undermine osteoporosis treatment.
tetracycline + zinc
Zinc forms a chelate with tetracycline in the gastrointestinal tract, modestly reducing absorption of the antibiotic. The interaction also reduces zinc absorption. Doxycycline is much less affected.
tetracycline + calcium
Calcium binds to tetracycline in the gut, forming an insoluble chelate that the intestine cannot absorb. Dairy products, calcium supplements, and calcium-based antacids can sharply reduce how much tetracycline reaches your bloodstream, which can drop levels below what is needed to treat the infection.
alendronate + coffee
Coffee (and orange juice) sharply reduce the absorption of alendronate, an oral bisphosphonate whose baseline absorption is already very low. Taking the tablet with coffee instead of plain water can cut the absorbed amount enough to make the dose ineffective for protecting bone.
prednisone + calcium
Glucocorticoids like prednisone impair intestinal calcium absorption and increase urinary calcium loss, contributing to a negative calcium balance and accelerated bone loss. This is a depletion-and-displacement effect, not a chemical interaction in the gut, and it is why calcium and vitamin D are treated as the foundation of bone protection during long-term steroid therapy.
itraconazole + grapefruit
Grapefruit juice can reduce the absorption of itraconazole capsules in healthy-volunteer studies, lowering antifungal blood levels. The likely mechanism is a rise in gastric pH that interferes with the capsule's dissolution, which outweighs grapefruit's usual CYP3A4-inhibiting effect.
copper + iron
Copper and iron work together in red blood cell production: copper-dependent ceruloplasmin is required to mobilize iron for hemoglobin, so inadequate copper can cause an anemia that mimics iron deficiency and does not respond to iron alone.
vitamin a + zinc
Zinc is required for the liver to synthesize retinol-binding protein, the carrier that moves vitamin A from liver stores into the bloodstream. When zinc is low, circulating vitamin A can stay low even though liver stores are adequate, and in deficient populations supplementing the two together corrects vitamin A status more reliably than vitamin A alone.
black tea + iron
Black tea is rich in polyphenols (tannins) that bind iron in the digestive tract. When the two are taken together, less of the iron your body needs gets absorbed.
curcumin + fat
Curcumin is a lipophilic molecule with very low water solubility, and dietary fat improves its dissolution and incorporation into bile-acid micelles for intestinal absorption. Taking curcumin or turmeric with a fat-containing meal, and using lipid-based formulations, raises its plasma exposure compared with intake on an empty stomach.
iron + vitamin a
Vitamin A and beta-carotene appear to improve absorption of non-heme iron from plant foods by forming soluble complexes with iron that keep it from binding to phytates and polyphenols in the gut. In controlled human absorption studies, adding vitamin A to a grain-based meal increased the amount of iron absorbed.
fat-soluble vitamins + dietary fat
Vitamins A, D, E, and K depend on bile-driven micelle formation in the small intestine to be absorbed, and that process is triggered by dietary fat. Taking these vitamins with a fat-free meal or on an empty stomach reduces how much you absorb, while taking them with a meal that contains some fat improves absorption. Controlled studies in vitamin D show meaningfully greater absorption when the supplement is taken with fat.
levothyroxine + soy
Soy protein and isoflavones can bind to levothyroxine in the gut and reduce how much of the dose is absorbed, which can raise TSH and, in some people, increase the dose needed to stay in range. The effect is most relevant with large, variable soy intake taken close to the dose, and is best documented in infants fed soy formula.
liothyronine + calcium
Calcium salts (carbonate, citrate, acetate) can bind thyroid hormone in the gut and reduce absorption of liothyronine (T3), much as they do with levothyroxine. Taking them together can blunt the effect of the dose and lead to less reliable thyroid replacement.
liothyronine + iron
Iron salts (ferrous sulfate, fumarate, gluconate, bisglycinate) can bind thyroid hormone in the gut and reduce absorption of liothyronine, similar to the well-documented interaction with levothyroxine. Taking them at the same time can blunt the T3 effect and let TSH drift upward.
black tea + thiamine
Black tea contains antithiamine factors - polyphenols such as tannins and chlorogenic acid - that can oxidise thiamine (vitamin B1) into biologically inactive forms in the gut before it is absorbed. Heavy habitual tea consumption has been linked to lower thiamine status, mainly in people whose dietary B1 intake is already marginal. For most well-nourished adults the effect is modest.
green tea + iron
Green tea polyphenols, especially the catechin EGCG, bind non-heme iron in the gut and form insoluble complexes that the intestine cannot absorb. The effect is most pronounced when green tea is consumed together with an iron supplement or an iron-rich plant meal, and it can be blunted by spacing the two apart and by pairing iron with a vitamin C source.
dairy + digoxin
Dairy is a dietary source of calcium, and calcium status modestly influences how digoxin acts on the heart. The dietary-dairy effect is minor on its own; the more relevant scenario is large stacked calcium loads (dairy plus supplements plus calcium-containing antacids) or intravenous calcium in a hospital setting. Milk proteins may also slightly reduce digoxin absorption from oral doses, but the effect is small and usually not clinically meaningful.
psyllium + metformin
Psyllium's viscous gel can slow and reduce metformin absorption when taken together, potentially blunting its glucose-lowering effect, while psyllium's own action lowers glucose — making net blood-sugar effects variable.
psyllium + warfarin
Psyllium is a soluble fiber that forms a viscous gel in the gut, and it was long suspected of trapping warfarin and slowing its absorption. However, the limited human evidence available — a pharmacokinetic study and the monographs that cite it — found that psyllium does not measurably change warfarin's blood levels or its effect on the INR. Because warfarin has a narrow safety margin, keeping fiber intake steady and spacing the doses remains sensible, but a clinically meaningful interaction has not been demonstrated.
oat fiber + statins
Oat fiber is rich in beta-glucan, a soluble fiber that forms a viscous gel in the gut. Taken at the same time as a statin, this gel can bind the statin tablet and slow its absorption, potentially blunting some of the cholesterol-lowering effect. The evidence is mechanistic and based largely on animal data; separating the two in time appears to resolve the conflict.
glucomannan + metformin
Glucomannan is a highly viscous soluble fiber that swells in the gut and can slow or reduce the absorption of medications taken at the same time, including metformin. Glucomannan also has its own modest glucose-lowering effect that may add to metformin's, so spacing the two apart and watching your blood sugar is sensible.
turmeric + black pepper
Piperine, the active alkaloid in black pepper, slows the gut and liver enzymes that normally inactivate curcumin (the main bioactive in turmeric). Taking the two together substantially increases how much curcumin reaches the bloodstream, which is why piperine is one of the most common absorption enhancers in turmeric supplements. The same enzyme effect can also raise levels of some prescription drugs, so concentrated daily supplement doses warrant a pharmacist check for people on chronic medications.
alcohol + folate
Chronic alcohol use causes folate deficiency through several mechanisms: it inhibits the reduced folate carrier in the intestine (blocking absorption), reduces the liver's uptake and storage of folate, and increases urinary folate loss. Folate depletion in turn accelerates alcohol-induced liver injury and disrupts one-carbon metabolism and DNA methylation.
alcohol + zinc
Chronic alcohol use lowers the body's zinc through reduced intake, impaired intestinal absorption, increased urinary loss, and altered zinc transporters (notably ZIP14). The relationship is bidirectional: zinc deficiency in turn worsens alcohol-related liver injury by weakening the intestinal barrier, allowing more bacterial endotoxin to leak into the portal blood, and reducing the liver's antioxidant defenses.
lycopene + fat
Lycopene is a fat-soluble carotenoid whose absorption depends on incorporation into bile-acid micelles, which require dietary fat. Human studies show that eating lycopene-rich foods with a fat source — such as olive oil or avocado — substantially increases how much lycopene reaches the bloodstream.
peppermint tea + iron
Peppermint tea is rich in plant polyphenols and tannins (including rosmarinic acid) that bind non-heme iron in the gut, forming insoluble complexes the body cannot absorb. Human studies show peppermint tea substantially reduces non-heme iron absorption from a meal, placing it among the stronger natural inhibitors. Heme iron from meat, poultry, and fish is not affected.
levothyroxine + fiber
Dietary and supplemental fiber can bind levothyroxine in the gut and modestly reduce how much of each dose is absorbed. When fiber intake is high or variable around the time of dosing, this can nudge TSH upward and make a stable dose harder to settle on. The effect is real but generally modest, and it is managed mainly by timing and consistency rather than by avoiding fiber.
tempeh + levothyroxine
Tempeh is a fermented soybean cake rich in soy protein, and soy protein binds levothyroxine in the gut and reduces how much of the thyroid hormone is absorbed. Fermentation lowers isoflavone bioavailability but leaves the soy protein intact, so the absorption interference remains. Taken consistently close to the dose, this can lower thyroid hormone levels enough to push TSH out of its target range.
levothyroxine + coffee
Coffee can reduce how much levothyroxine you absorb when the two are taken at the same time. Chlorogenic acids and other compounds in coffee appear to bind the hormone in the gut, and coffee can also speed gastric transit, leaving less time for the tablet to dissolve. The effect largely disappears when the dose and the coffee are separated by enough time.
omeprazole + vitamin b12
Omeprazole suppresses gastric acid, which is needed to release vitamin B12 from dietary proteins before it can bind intrinsic factor and be absorbed. With long-term use this can lower serum B12 and, over time, contribute to deficiency. Supplemental (crystalline) B12 is not affected because it does not depend on stomach acid.
coffee + vitamin b1
Coffee and tea were historically labeled antithiamine beverages, but later biochemistry walked the claim back: chlorogenic and caffeic acids do not destroy thiamine under physiological conditions, and the real activity comes from polyphenol oxidation products and tannins, which are lower in coffee than tea. The net effect on thiamine status is modest and unlikely to matter for well-nourished people; it becomes relevant only on a marginal diet or in groups already prone to deficiency.
phenytoin + calcium
Calcium-containing supplements and antacids can bind phenytoin in the gut and lower how much of the drug is absorbed when the two are taken together, which can reduce phenytoin's blood level. Separately, long-term phenytoin use can reduce calcium absorption by speeding up the breakdown of vitamin D, which is relevant to bone health over time.
probiotics + antifungals
Systemic antifungals (such as fluconazole, itraconazole, amphotericin B, and the echinocandins) can kill yeast-based probiotics such as Saccharomyces boulardii, blunting their benefit. Bacterial probiotics like Lactobacillus and Bifidobacterium are generally unaffected, because their cell structure differs from fungi.
ketoconazole + calcium
Calcium carbonate antacids and large calcium supplements neutralize stomach acid and raise gastric pH. Oral ketoconazole needs an acidic stomach to dissolve and be absorbed, so taking the two together can lower ketoconazole blood levels and reduce its antifungal effect.
gabapentin + antacids
Aluminum- and magnesium-containing antacids reduce the amount of gabapentin absorbed when the two are taken at the same time. The effect is mechanical (the antacid interferes with gabapentin's intestinal uptake) rather than acid- or pH-related, so it can be largely avoided by spacing the two doses a couple of hours apart and taking gabapentin after the antacid.
alendronate + calcium
Calcium binds alendronate in the gut and forms an insoluble complex, sharply reducing absorption of an already very poorly absorbed bisphosphonate. Taken together, the calcium can leave the osteoporosis drug clinically ineffective.
magnesium + glycine
Magnesium and glycine are commonly combined as magnesium bisglycinate, a chelate whose clearest benefit is being gentle on the gut and improving adherence, rather than dramatically higher absorption.
levofloxacin + calcium
Calcium binds (chelates) levofloxacin in the gastrointestinal tract, modestly lowering the antibiotic's peak blood level. Total drug exposure over the dosing interval is largely preserved, so the effect is smaller than with iron, magnesium, or aluminum. Separating the doses by a couple of hours avoids the interaction.
magnesium + zinc
At high supplemental doses, zinc and magnesium can each modestly reduce the other's absorption in the gut — and the better-documented direction is zinc lowering magnesium absorption, not the reverse. The effect is minor and dose-dependent; ordinary multivitamin amounts rarely matter.
curcumin + quercetin
In laboratory intestinal-cell models, quercetin slows the gut and liver enzymes (UDP-glucuronosyltransferase and CYP3A4) that normally break curcumin down quickly, which raised curcumin's measured permeability across the cell layer. Both polyphenols also act on overlapping anti-inflammatory and antioxidant pathways. The evidence is mechanistic and limited to in vitro work — no human trials have confirmed a real-world bioavailability or anti-inflammatory benefit from combining them.
omeprazole + iron
Omeprazole reduces absorption of nonheme (plant and supplemental) iron by raising stomach pH, which hinders the conversion of ferric (Fe3+) iron to the absorbable ferrous (Fe2+) form. Population data link long-term proton pump inhibitor (PPI) use with a higher risk of iron deficiency. A second, hormonal mechanism involving hepcidin and ferroportin has been proposed but rests on laboratory and animal work, not human outcomes.
oolong tea + iron
Oolong tea is partially oxidised and contains the same families of iron-binding polyphenols found in green and black tea, including catechins and theaflavins. These polyphenols can bind non-heme iron in the gut and lower how much is absorbed when tea is taken with iron-rich meals or supplements. The effect is well documented for green and black tea; for oolong specifically it is a reasonable extrapolation of the same mechanism rather than a directly measured result.
smoking + vitamin b12
Cigarette smoke can lower usable vitamin B12 by converting active coenzyme forms to inactive cyanocobalamin and by impairing gastric absorption, creating a low-grade nutritional draw rather than a dangerous reaction.
milk + levodopa
Milk and other protein-rich foods supply large neutral amino acids that compete with levodopa for the LAT1 transporter at the gut wall and blood-brain barrier, reducing levodopa absorption and brain uptake and weakening symptom control.
dairy + fluoroquinolones
Calcium and other metal ions in dairy products bind oral fluoroquinolone antibiotics in the gut, forming poorly absorbed chelate complexes that lower the amount of antibiotic reaching the bloodstream.
whey protein + iron
Whey protein is usually consumed alongside calcium-rich milk minerals, and calcium competes with iron for absorption in the gut. When taken at the same time, a whey-plus-iron serving can modestly lower how much iron you absorb. The effect is largely driven by calcium, is generally modest, and is easily offset by taking a vitamin C source with your iron.
edamame + levothyroxine
Edamame is whole young soybeans. Soy protein and isoflavones can bind levothyroxine in the gut and modestly reduce how much is absorbed if the two are taken close together. The evidence is limited and mixed: case reports and one pharmacokinetic study suggest a small reduction in absorption, while the only randomized crossover study found no significant effect. Any impact is best avoided simply by separating the dose from soy-rich meals.
coffee + iron
Coffee contains chlorogenic acid and other polyphenols with galloyl groups that bind non-heme iron in the gut, forming poorly soluble complexes the intestine cannot absorb. Drinking coffee with or shortly after an iron-rich meal or supplement meaningfully reduces how much non-heme iron you take up.
caffeine + vitamin d
Higher caffeine intake is weakly associated with lower vitamin D status. In cell studies caffeine reduces vitamin D receptor (VDR) expression in bone-forming cells, and a large NHANES cross-sectional analysis links higher caffeine intake to a modestly greater chance of low serum 25-hydroxyvitamin D. The effect is small and matters most for people who already have low vitamin D, low calcium intake, and high bone-loss risk (for example, postmenopausal women). It is not an absorption-level interaction, so there is no need to separate the timing of a vitamin D supplement from coffee.
tofu + levothyroxine
Tofu is one of the most concentrated sources of soy protein in the diet. Soy protein binds levothyroxine in the gut and reduces how much of the drug is absorbed, and case reports describe TSH rising when soy-heavy foods like tofu are added to a previously stable levothyroxine regimen. The effect is modest in most adults but clinically relevant in sensitive patients, and it is managed by timing.