I didn't notice that this product is made of "Oyster Shell" calcium, and I usually take carbonate or citrate calcium from other sources because unfortunately medical boards have warned against taking calcium with "natural oyster shell" as it "might" contain heavy metals and lead. I'll make sure I read the small print next time I order calcium online. I give it 4 stars because the amount of elemental calcium you get compared to the cost is pretty good, but these are hard tablets and not liquid filled capsules that easily absorb. Next time I'll make sure I read the fine print better.