Timing & Absorption
Why the timing of a dose can matter as much as the combination itself -- and the specific, sourced separation windows currently on record.
Some supplements and medications do not have a documented direct interaction, but can still interfere with each other's absorption if taken too close together. Minerals in particular (calcium, iron, magnesium, zinc) can bind to certain medications in the gut and reduce how much the body actually absorbs. Separating doses by a period of time is often the practical fix -- rather than avoiding the combination altogether.
The rules below are only ever published when a specific, reviewed source documents a specific window -- never a generic "take supplements two hours apart" rule of thumb. If a pair you are wondering about is not listed here, check it directly -- its interaction page will show any timing consideration we have reviewed for it, or say plainly that none has been reviewed yet.
Reviewed rules
Current timing & spacing rules
Each rule links to the substance page it applies to and shows the evidence quality behind it.
Leave about 240 minutes between Calcium and Levothyroxine.
Evidence: HighCalcium pageLeave about 240 minutes between Iron and Levothyroxine.
Evidence: HighIron page
For interaction classifications rather than timing, see Important Interactions. For general supplement/medication guidance, see Supplement Safety and Medication Safety.