The more important question that needs to be answered first is, why did the doctor recommend a calcium supplement to you in the first place? Most people in dairy consuming countries get too much calcium, not too little. Dairy is quite high in calcium but quite deficient in magnesium. The mineral most people lack who have a "calcium" problem is magnesium, not calcium. Most times a doc recommends calcium supplementation especially for women is for bone strength and development. But magnesium, as well as some trace minerals such as boron, are also essential for bone strength. In bones, calcium and magnesium are in electrical balance opposing one another in maintaining the strength of the bone. Too much calcium will leach magnesium out of the body (and vice versa) and out of the bones, making them brittle. This is also a result of using forms of calcium that are not good ones, such as hydroxyapatite. This is also a problem with drugs for bone strength, such as Boniva (which is why in your dentist's office as you sit there you're usually looking at a sign asking you to tell them if you're taking one of these drugs so they don't break something). Complicated, eh? A big part of the problem particularly in the US where bone strength is a problem greater than it should be given our overall wealth and food availability is that Americans don't like green leafy veggies that much. But that's where you get the best absorbed calcium, and eat those stems on your broccoli, folks, the harder parts are full of minerals, and also a balance of good quality magnesium. But if you're one of the many women who have been told dairy is good for your bones, you're leaching out your magnesium and actually building weaker bones. Also, hormones play a factor in this as well. So, both forms of calcium you mention are fine provided you actually need a calcium supplement. Citrate is the most often recommended because minerals need an acidic stomach to be properly broken down and absorbed -- which brings up another problem, the large number of Americans on long-term treatment with acid suppressing drugs for digestive problems, the stomach needs that acid to break down both minerals and protein -- and citric acid is good for this. Some just drink some orange juice with it for the same purpose. The ones to avoid are mainly the ones that are actually rocks -- calcium carbonate, oyster shell calcium, as two examples -- too hard to break down for most people. You can also buy a calcium/magnesium combo with the proper 2:1 ratio of calcium to magnesium. But again, did your doctor do a test that shows you lack calcium? Did he do one that checks magnesium? I bet not. So again, the first question is, do you have a particular problem that makes this supplement a requirement for you, and if so, both forms you have mentioned will probably be absorbed well by you. But there are many different forms, and many are good. Different people will absorb different ones better than others, but without some form of nutritional testing on a regular basis you won't really know which is best for you. We can't explain all the forms of calcium because there are so many what are called chelates, meaning the calcium is calcium but the form is made different by both the sourcing and what is added (the chelate). Now, something you can also do if you're concerned about bone health is to buy a bone formula or a balanced calcium formula -- there are many good ones in a better health food store. They will contain many of the nutrients that go into building strong bones. The most commonly used form, though, for good absorption is the citrate form, and the cheapest form are the rocks (by the way, for those who use Tums regularly that's one of the forms made of rock, and sometimes when these can't be broken down they get deposited where you don't want them, such as stones in your organs).