Calcium carbonate. The cheapest supplement, calcium carbonate is also the highest in elemental calcium: 40 percent. This is the form of calcium found in Tums and many generic versions. It has one big drawback: It dissolves slowly in your stomach, so you may not get the full benefit of all the calcium.
Calcium phosphate (also called tribasic calcium phosphate). This form is 39 percent elemental calcium. You don't need the extra phosphorus that comes with these tablets—skip them.
Calcium citrate. This is the form many doctors and nutritionists recommend. Calcium citrate is only 21 percent elemental calcium, and it's relatively expensive. On the big plus side, it dissolves easily even if you don't have much stomach acid, so you're more likely to absorb all the calcium before the pill passes out of your stomach. Many people naturally produce less acid as they age, so calcium citrate is a good choice for older adults. It's also good for people taking acid-blocking drugs such as ranitidine.
Calcium lactate. This form is found in many generic calcium supplements. It has only 13 percent elemental calcium and is relatively expensive. On the other hand, it dissolves easily even if you're low on stomach acid, so, like calcium citrate, it's a good choice for older adults and people who take acid-blocking drugs. Calcium gluconate. This form is also found in many generics, but it has only 9 percent elemental calcium. It's not a very good choice.
Calcium glubionate. This is a concentrated syrup form that contains 6.5 percent elemental calcium. You'd need to take 12 teaspoons a day to get 1,000 mg of calcium. Calcium glubionate is on the expensive side. Take it only if your doctor suggests it.