The educational campaign to debunk the old myth of poinsettias being poisonous has been quite successful. Most people have gotten the message that poinsettias won’t kill humans if consumed.
Poinsettias were considered poisonous as recently as 1970, when the Food and Drug Administration issued a press release saying one poinsettia leaf could kill a child. Apparently the myth started in 1919 when it was rumored that a 2-year-old child in Hawaii died after chewing on a poinsettia leaf. The child did indeed die, but from causes totally unrelated to eating the poinsettia leaf. But that was enough for authorities to declare the poinsettia poisonous for the next 60 years. There are dozens of cases of people attempting suicide by ingesting poinsettia leaves.
Ohio State University tackled the poinsettia poison issue in 1971 with an extensive study in which rats were fed large quantities of poinsettia leaves, with no adverse effects, other than weight gain. Their research was the first scientific study that exonerated the poinsettia.
In 1974, a Canadian horticulturist ate the leaves of a poinsettia plant in front of members of the press to prove that poinsettias were non-poisonous, and to help publicize that poinsettias were safe.
So poinsettias won’t kill humans, but what about pets? What if cats and dogs chew or consume poinsettia leaves? 
Poinsettias have a toxicity rating of “Low.” The low-toxicity, rather than no-toxicity, is because the milky sap can irritate the mouth and cause rashes, nausea, vomiting and diarrhea. The universities of Purdue, Cornell, Washington State U, and many others reiterate the non-lethal nature of poinsettias, including for pets. If cats and dogs eat poinsettias, they might develop mouth irritation, intestinal disturbance and vomiting, but quickly recover. Some individuals have no adverse effects from poinsettia consumption. There have been no deaths from pets eating poinsettias.
The Veterinary Teaching Hospital at Colorado State University answers the question in great depth: “Commercially available poinsettias are not poisonous to dogs and cats or people. Other members of this Euphorbia plant family are poisonous as they can induce severe skin and mucous membrane irritation in some people.
“Poinsettia’s white latex sap in the leaves and stems is mildly irritating to the mucous membranes of the mouth and in some animals will induce excessive salivation and vomiting if the plant parts are swallowed. The wide variety of hybrid poinsettias available today have very little toxicity compared to the parent species. Unless a cat or dog eats a considerable amount of the poinsettia plant, the animal is not likely to show any effects. The modern day poinsettias sold commercially during the holiday season are not a serious risk to animals. This does not mean that if a pet has a habit of chewing on plants that it should be allowed to!”
