http://www.snopes.com/critters/crusader/greenies.asp
Origins: Greenies are a brand of green, toothbrush-shaped dog treats developed to keep dogs' teeth and gums healthy and control "doggie breath." Dogs have to do a lot of chewing to get through one of the biscuits, which helps to keep their teeth clean and control tartar, and the ingredients are touted as providing better nutrition and bad breath control than other alternatives (such as rawhide chews).
In November 2005, the above-quoted e-mail began circulating, warning readers that Greenies did not appear to be fully digestible. A 15 November 2005 consumer investigation by Seattle's KIRO-TV reported that Greenies and similar products could "pose a real danger to dogs." KIRO offered three accounts from dog owners whose pets had to undergo veterinary treatment for intestinal obstructions after being fed Greenies or other teeth-cleaning biscuits; two survived after surgery, and the third died. All in all, they found nine people who claimed their dogs had died after eating Greenie-like
products.
S&M NuTec, the company that produces Greenies, sent KIRO a statement asserting that "millions of Greenies are sold and enjoyed by dogs, every week without incident," and that "though injurious incidents are rare, more often than not, the pet is not fed according to our feeding directions." The warning label on packages of Greenies does caution consumers, "As with any edible product, monitor your dog to ensure the treat is adequately chewed. Gulping any item can be harmful or even fatal to a dog," but as most every pet owner knows, trying to monitor your dog (or cat) the entire time it's chewing anything is a near-impossible task.
A number of different pet products — from teeth-cleaning chews to doggie biscuits to chew toys — can potentially injure or sicken dogs under certain conditions and circumstances; we don't yet have enough information to know whether Greenies pose a greater than normal danger to dogs under ordinary use than other similar products, or whether we're just seeing a confluence of exceptional anecdotal reports.