Your body needs to be able to absorb calcium for it to be effective. Taking calcium supplements in small doses (500 mg or less) during meals is the best way to ensure that your body can absorb the calcium. While calcium supplements can be taken at any time of the day, the best time to take them will depend on your individual needs. Factors such as dosage, other medications, and the type of calcium you're consuming will all affect when you should take it.
Calcium is better absorbed when taken with food because the stomach acid produced helps with absorption. The exception is calcium citrate, which can be taken without food and still be absorbed.
