Safety has not been established for any dose of oleander. Peruvoside, a heart-active substance in yellow oleander kernels (similar to the drug digoxin), has been studied at 1.8 to 3.2 milligrams by mouth, as an initial dose, followed by an average daily dose of 0.6 milligrams per day for congestive heart failure.
Oleander is not recommended for use in children due to risk of toxicity or death and lack of scientific data.
People with allergy/hypersensitivity to oleander or other cardiac glycosides such as digoxin or digitoxin may have reactions to oleander. Skin contact with sap from oleander leaves may cause rash.
Common oleander contains a strychnine-like toxin and a heart-active cardiac glycoside substance (similar to the prescription drug digoxin) that may cause the heart to beat rapidly, abnormally, or to stop beating. Common oleander has been used as rat poison, insecticide, and fish poison and is toxic to mammals including humans. Animals (sheep) have died after eating as little as two to three leaves of Nerium oleander (common oleander). Children may die after eating a single leaf of common oleander. Eating the leaves, flowers, or bark of common oleander may cause nausea, vomiting, stomach cramps, pain, fatigue, drowsiness, unsteadiness, bloody diarrhea, abnormal heart rhythms, seizures, liver or kidney damage, or unconsciousness. Death may occur within one day. Reports of toxicity and deaths in children and adults have been reported for decades in Australia, India, Sri Lanka, and the United States.
Fruits of Thevetin peruviana (yellow oleander) are thought to be even more toxic to mammals, including humans. Based on human studies of intentional overdose (suicide attempts), eating eight or more seeds of yellow oleander may be fatal. Additional side effects of oleander ingestion include irritation and redness of lips, gums, and tongue, nausea, vomiting, depression, irritability, fast breathing, sweating, stomach pain, diarrhea, headache, confusion, visual disturbances, and constricted pupils. Abnormal blood tests, including tests of liver and kidney function (potassium, bilirubin, creatinine, and blood urea), have been reported in humans.
It is possible that plants grown in the same soil as oleander plants or in soil exposed to oleander may contain trace amounts of oleander.