Iron and calcium are the classic example of two supplements that are individually fine but compete when taken at the same time. Neither is dangerous; the issue is that calcium blunts how much iron you absorb. The fix is not to avoid either one, but to put a few hours between them.
Why they compete
Calcium and iron use some of the same machinery to cross from the gut into the bloodstream, and calcium tends to win. Take a big calcium dose, or a calcium-rich meal, at the same time as your iron, and a chunk of that iron simply does not get absorbed. If you are taking iron to correct a deficiency, that lost absorption is the difference between fixing the problem and treading water.
The simple fix: separate them
Take your iron at a different time of day from your main calcium, ideally a couple of hours apart. A common pattern is iron earlier in the day with a source of vitamin C to boost it, and calcium with an evening meal. The same logic applies to a calcium-containing antacid or a glass of milk: keep them away from your iron dose rather than alongside it.
The wider pattern
Iron is fussy about company in general. As well as calcium, it absorbs less well alongside coffee, tea, and high-fibre supplements, and it competes with zinc too. The reliable approach is to give iron its own slot with vitamin C, and let the things that interfere with it sit elsewhere in the day. This is also why iron and thyroid medication need separating, covered in the levothyroxine guide.
To check iron, calcium, or both against any medication you take, the free checker will show you the result and the reasoning.