It’s a myth. Poinsettias leaves are not poisonous. That’s the consensus of government agencies, health centers, veterinary and plant and flower organizations. Poinsettias are not toxic and do not pose a health threat to children or pets. If ingested in a large quantity, the leaves may cause stomach upset or mild diarrhea.
The plant was named for amateur botanist and gardener, Joel Roberts Poinsett (1779-1851) who first saw the shrub in Mexico.
It was shown at the Philadelphia Flower Show in 1829.