U.S. Doctors' Clinical Calcium, Magnesium + Vitamin D3 tablets are made with a blend of calcium carbonate and calcium citrate, which are two forms of absorption-friendly calcium. Since calcium is only available through nutrition and supplementation, it's important to supply your body with sufficient calcium levels to support bone health. Our innovative formula includes magnesium and vitamin D3 to help the body utilize calcium effectively. Magnesium and vitamin D3 also play a vital role in muscle and nerve function, and may promote relaxation and improve sleep quality.