There seems to be a sudden burst of calcium products on the market for pets, but the fact is most dogs and cats don't need them, and they could be harmful!
Though calcium is a necessary mineral for humans and pets, most of us don't need supplements because we get calcium in our food. And we all have to be careful not to take too much of the stuff because hypercalcimia can cause fatal conditions, such as heart failure, renal failure, and seizures. These conditions are very rare, but frankly, I worry about supplement manufacturers advertising calcium supplements for dogs and cats when they probably are not needed.
If you feed your pets commercially available canned or dry pet food there is appropriate calcium for most dogs and cats already in the food, as it required by the U.S. Department of Agriculture Food and Drug Administration.
If, on the other hand, you feed your pets a homemade diet, a supplement might be recommended, if natural foods, such as broccoli, spinach, salmon, ground egg shell, or other high-calcium foods are not included in your pets' diets. A blood test administered by your vet will let you know if your pet is calcium deficient as well with other nutrient deficiencies, and I strongly advise you to have at least yearly blood tests conducted if you do feed raw or home-cooked diets to your pets.
Ground eggshell is one of the most natural forms of calcium supplement available, but eggs might not be your best friend or your pet's and it takes a special machine to get it ground fine enough for it to be digestible.
Pet's Friend makes a 5-star pure eggshell powder called Eggshellent Calcium which comes in a 16 ounce container. Customers mix this with their pet's food and are so pleased with the results that some of them take it themselves. Although Eggshellent Calcium is made for pets, there is nothing else in the contents but ground eggshells, so it won't hurt people either. The eggs are sourced in the U.S.
Again, make sure to get the okay from your vet to give a calcium supplement to your pet. You may or may not need one yourself!