According to the National Institutes of Health, if taken properly and in appropriately small doses, eucalyptus leaf is not poisonous to humans. However, any amount of eucalyptus leaf can be harmful and potentially fatal to cats, dogs and horses.
Eucalyptus leaves contain an essential oil called eucalyptol and, if consumed in small quantities by adult humans, are unlikely to be harmful. The National Institutes of Health recognizes that eucalyptus oil and leaves can be safely used in a variety of medical treatments, including in vaporizer treatments to cure congestion. However, the NIH cautions that eucalyptus oil should be diluted before being consumed orally or applied directly to skin. While generally safe for humans, domesticated animals should not be exposed to eucalyptus leaves or oils. Of course, these smoky-green leaves are not completely poisonous to all animals — the Australian koala subsists entirely on eucalyptus, eating multiple pounds of the nutrient-poor plant each day.