Supplement Interactions

What Competes

Which pairings genuinely interact, the mechanism behind each, and how strong the evidence really is.

Most supplement interaction content presents every pairing at the same level of confidence, which is how you end up with an internet where everything interacts with everything and none of it helps you decide. Every page here carries an evidence tier: well documented, human clinical evidence, human evidence limited, mechanism only, or commonly claimed with weak evidence. The tier is the point. It tells you how much weight the finding can carry before you read a word of it.

Minerals that compete

Minerals share absorption pathways, so taking one at a high dose for a long time in isolation is the pattern that causes problems.

Zinc and Copper

Well documented. Sustained high-dose zinc without copper is a known cause of copper deficiency.

Iron and Calcium

Human clinical evidence. Real in a single meal, much smaller across a whole diet.

Iron and Zinc

Human clinical evidence. A ratio question, and mostly absent once the minerals arrive in food.

Magnesium and Calcium

Commonly claimed, weak evidence. The 2 to 1 ratio is not what people think it is.

Pairings that help

Not every interaction is a warning. Some combinations are the reason a nutrient works at all.

Vitamin C and Iron

Human clinical evidence. The one pairing here where together is the goal.

Vitamin D and K2

Human clinical evidence. Where the calcium goes once vitamin D has helped you absorb it.

Vitamin D and Magnesium

Human clinical evidence. The enzymes handling vitamin D are magnesium dependent.

Selenium and Thyroid

Well documented. A meta-analysis of 35 studies, and a mineral with a real ceiling.

Caffeine and L-Theanine

Human clinical evidence. The pairing behind the calm-focus effect people chase.

Worth asking your prescriber about

Compounds active enough to help are active enough to interact. That is not an argument against them, it is an argument for your prescriber knowing.

Berberine and Blood Sugar Medication

Human clinical evidence. Overlapping pathways, plus a documented effect on drug-clearing enzymes.

Calcium and Thyroid Medication

Well documented. A four-hour spacing question that is easy to get wrong.

Omega-3 and Blood Thinners

Human evidence limited. More nuanced than the warnings suggest, and real where it is real.

Probiotics and Antifungals

Mechanism only. A timing rule that rests on reasoning rather than trials.

When the supplement changes the test

One entry, because it is the interaction people are least likely to suspect and most likely to be harmed by.

Biotin and Lab Tests

Well documented. A hair and nail supplement that can make your thyroid results say something untrue.

No page here sells the supplement it is warning about. Where a real caution exists it is stated plainly, and where a widely repeated interaction turns out to rest on very little, that is stated just as plainly.

14 pages published so far. More are being added.

Know what interacts with what

Occasional notes on the pairings worth knowing about, the mechanisms behind them, and how strong the evidence actually is.