Puppies - calcium doesn't cause constipation, as far as I know. Iron is generally more of a problem. Caltrate - I worked in a pharmacy, and caltrate is probably the crappiest calcium you can get, lol. Sorry, but it really is. It's poorly absorbed, and is about one step up from eating bits of chalk. There are lots of great supps on the shelves, just ask around a bit! Remember, a lot of pharmacies get bonuses for recommending things like caltrate (ours did!)