Vitamin D is needed for calcium absorption. The vitamin doesn't have to be taken at the same time as the mineral, though some supplements combine both nutrients.
Some calcium supplements, such as calcium carbonate, are best taken with food because the stomach acid released during digestion helps absorption. Calcium citrate is different. It can be absorbed on an empty or full stomach. This type of calcium is often suggested for those over 50 who tend to have less stomach acid than younger people.
Mineral oil and some laxatives block calcium absorption. Tannins in tea and the fiber in wheat bran can also interfere with absorption. Be sure to talk to your doctor about all medicines and dietary supplements you take, as some supplements can reduce or increase the effects of prescription and nonprescription drugs.