The honest answer to "what supplements should I not take together?" is reassuring: very few combinations are actually dangerous. Most of the clashes people worry about are not about harm at all, but about competition for absorption, and the fix is almost always timing rather than avoidance. The genuinely off-limits combinations tend to involve a supplement and a prescription medicine, not two supplements.
The common clashes (which are really just timing)
These are the pairs worth spacing out by a couple of hours, not because anything bad happens, but because taken together one blunts the other:
- Iron and calcium. Calcium reduces iron absorption, so keep them apart if you are taking iron to fix a deficiency.
- Iron and zinc. They compete for the same uptake route; space them out if you take both.
- Iron with coffee or tea. The tannins cut iron absorption, so a cup right alongside your iron works against it.
- Zinc and copper, over the long term. Months of zinc without copper can deplete copper; the fix is matching them, not avoiding zinc.
- Large doses of calcium and magnesium at once. A minor competition at high doses; ordinary doses are fine together.
The combinations that are genuinely off-limits
The real "do not combine" cases are mostly about supplements stacked on top of medication, not supplement-with-supplement. The clearest are 5-HTP or St John's Wort with an antidepressant, where the serotonin effect can stack dangerously, and St John's Wort with a long list of prescriptions it speeds the clearance of. Those belong to the medication guides rather than here, because the risk comes from the drug side of the pairing.
The simple version
Among supplements alone, think timing, not danger. Give iron its own slot, match zinc with copper over the long haul, and do not stress about the rest. The combinations that genuinely matter almost always involve a medication, which is exactly what the free checker is built to catch. If timing is the main thing to get right, the timing guide lays out a simple daily pattern.