Avoid taking calcium supplements with foods that are too high in fiber. Fiber can bind to calcium, reducing the amount available to the body. Don't take more than 500 to 600 mg at a time, as the body can't absorb much more and it will be excreted in the urine. Calcium can also decrease the absorption of some medications, such as osteoporosis medications, thyroid medications, and some antibiotics.
Ask your doctor if your medications can interact with calcium, or to avoid risks, simply don't take them at the same time. Taking calcium and vitamin D supplements with thiazide diuretics may increase the chance of kidney stones forming. Consequently, it is reasonable to speculate that the effect of calcium supplements on the risk of kidney stones depends on whether the supplements are given with or between meals. I've heard that you have to wait an hour between taking calcium supplements and taking other supplements and medications.
