Many owners believe dogs eat grass to make themselves sick. Research suggests that's not usually the case: most dogs that eat grass seem perfectly well before and don't vomit afterwards. Wild canids eat plants too, and scientists think it may be an inherited behaviour, possibly once useful for clearing intestinal parasites.
Grass eating is mostly harmless, but watch for changes. A dog that suddenly starts eating grass frantically, gulping it and licking surfaces may be nauseous, and grass treated with pesticides or fertilisers, or toxic garden plants, can be dangerous.
