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 and calcium with an evening meal. Adding vitamin C to the iron is optional: it does increase absorption from a tablet, but trials found little or no difference in haemoglobin or ferritin, so there is no need to buy it specially. 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, dairy, eggs and high-fibre supplements. Zinc can compete with it when the two are taken together on an empty stomach, and magnesium may too, though the evidence for magnesium comes mostly from antacids. The reliable approach is to give iron its own slot and keep these a couple of hours away; for tea, coffee, eggs and dairy the NHS gives a two-hour gap after taking iron. Some antibiotics need a different gap again: take ciprofloxacin or doxycycline at least two hours before, or six hours after, any iron or calcium supplement. 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.