Supplements, food, and alcohol

Alcohol adds to the sedative effect of many medicines, stresses the liver, and changes how some drugs are absorbed and cleared.

Caffeine is a stimulant cleared by the CYP1A2 enzyme; drugs that block that enzyme can make coffee hit much harder.

Cranberry has been suspected of boosting warfarin's effect, though studies disagree.

The calcium in dairy binds some antibiotics and other drugs in the gut and blocks their absorption.

Grapefruit and grapefruit juice block the CYP3A4 enzyme in the gut wall, so more of certain drugs reaches the bloodstream.

Green tea contains caffeine and vitamin K and has been shown to lower blood levels of at least one beta blocker.

CoQ10 is chemically similar to vitamin K and may weaken warfarin.

Echinacea stimulates the immune system and inhibits some CYP enzymes.

Omega-3 fatty acids have a mild antiplatelet effect that matters mostly alongside blood thinners.

Garlic supplements can thin the blood slightly and interfere with at least one HIV drug.

Ginkgo is best known for adding to bleeding risk with blood thinners.

Ginseng has been reported to change the effect of warfarin and to lower blood sugar.

Glucosamine and chondroitin have been reported to raise INR in people taking warfarin.

Melatonin adds to drowsiness from sedating medicines and is cleared by the CYP1A2 enzyme.

St. John's wort is one of the most interaction-prone supplements: it induces CYP3A4 and P-glycoprotein, lowering levels of many prescription drugs.

Turmeric supplements are far more concentrated than the spice and are most often flagged alongside blood thinners.

Antacids raise stomach pH and contain metal ions that bind many drugs, cutting their absorption.

THC adds to the sedation of many medicines and is metabolized by CYP enzymes that other drugs block or induce.

CBD inhibits several CYP enzymes and has documented interactions with some seizure medicines and blood thinners.

Smoke (not nicotine itself) induces the CYP1A2 enzyme, lowering blood levels of some drugs; quitting can then push those levels up.

Calcium binds to several drugs in the gut and blocks their absorption; separating doses by a few hours usually solves it.

Several drugs lower folate, and folic acid can in turn change how some of them work.

Iron chelates with several antibiotics and thyroid medicine, and stomach-acid reducers lower how much iron you absorb.

Magnesium supplements and antacids bind to some antibiotics and bone drugs, and several common medicines change magnesium levels.

Potassium supplements and salt substitutes can push potassium dangerously high when combined with drugs that also retain potassium.

Common stomach-acid reducers and metformin lower B12 absorption over time.

Some medicines lower vitamin D levels; with thiazide diuretics, vitamin D can raise blood calcium too high.

High-dose vitamin E can inhibit platelet aggregation and add to the bleeding risk of blood thinners.

Vitamin K (in supplements and leafy greens) directly counteracts warfarin, so a steady intake matters more than avoiding it.

Zinc binds some antibiotics in the gut, and a few drugs increase zinc loss.