Oleander

Oleander is an evergreen shrub or small tree 2–6 m tall, branching from the base, with milky white sap. Leaves are opposite or in whorls of three, lanceolate to oblong, 5–21 cm long and 1–3.5 cm wide, entire, glossy dark green above with a prominent midrib. Bisexual flowers are borne in terminal cymes; the funnel-shaped corolla has five twisted lobes, usually pink to red, sometimes white or yellow, is fragrant, and bears a fringed corona in the throat. The fruit consists of two long follicles 8–20 cm bearing many seeds with a tuft of hairs. The whole plant contains poisonous latex. The chromosome number is 2n = 22.

According to Kew POWO, the native range of oleander runs from the Mediterranean to Myanmar, covering Mediterranean southern Europe and North Africa (Spain, France, Italy, Greece, Albania, Portugal, Morocco, Algeria, Tunisia, Libya), the Middle East and western Asia (Türkiye, Syria, Lebanon, Israel/Palestine, Iraq, Iran, the Arabian Peninsula, Afghanistan, Pakistan), and on to India, the West Himalaya, Nepal and Myanmar. It grows in dry stream beds and by water, from sea level to about 1,500 m. It is widely planted and naturalised in more than 69 other territories (the Americas, Australasia, the Pacific, tropical Asia). In Vietnam it is grown ornamentally along roads, in parks and gardens.

The whole plant contains milky latex and a mixture of cardenolide cardiac glycosides — the most powerfully heart-active class of plant compounds. The principal active is oleandrin (with its precursor oleandrigenin), together with neriine, neriifolin, digitoxigenin, oleandroside, nerioside, peruvoside, thevetin and thevetoxin. Structurally they have a steroid core with a C17 lactone ring and a sugar (usually digitoxose), closely resembling digitoxin from foxglove. Glycoside content varies with cultivar and season: red-flowered plants in flower contain more than white ones. Besides cardiac glycosides the plant has flavonoids, triterpenes (α-amyrin, lupeol), phytosterols and organic acids. Crucially, the toxin is heat-stable: drying, boiling or burning does NOT destroy the glycosides — even the smoke of burning plants is toxic, and vase water or nectar can also be contaminated.

The main toxic mechanism is inhibition of the Na⁺/K⁺-ATPase pump in heart-muscle cell membranes, raising intracellular Na⁺ and Ca²⁺ and blood potassium (hyperkalaemia), which triggers arrhythmias. Symptoms include nausea, vomiting, abdominal pain, diarrhoea, dizziness, blurred vision, bradycardia, atrioventricular block, ventricular tachycardia, ventricular fibrillation and death. Estimated lethal doses: adults 5–15 fresh leaves or ~3 g dried leaves; a single leaf can kill a child; the toxic blood level is 1–2 ng/ml and the fatal level about 20 ng/ml. Much literature documents poisonings and deaths from suicide, accident or folk-medicinal use (comprehensive review, Human & Experimental Toxicology 2020; review of fatal cases, 2025). Treatment comprises activated charcoal, atropine, temporary pacing and digoxin-specific Fab antibody fragments (DigiFab/Digibind), effective in reported cases (JACC 2025). Pharmacologically, oleandrin has been studied for cancer (PBI-05204, Anvirzel — phase I/II trials) and as an antiviral (including SARS-CoV-2, Plante 2021), but its therapeutic window is very narrow; a human iPS-cardiomyocyte study (2021) ranked arrhythmia potential as neriifolin > oleandrin > digitoxigenin, with beat arrest at only 300–600 nM. Because of this high toxicity, oleander must NOT be used as medicine: every folk use is dangerous.

Safety warning: NEVER eat or chew the leaves, stems or flowers, never use the plant as medicine, never drink vase water, and never burn it indoors; plant it only where young children and pets cannot reach. Poisoning can occur via nectar, vase water, food skewered on oleander sticks, or livestock browsing the leaves. If poisoning is suspected, call emergency services at once; do not induce vomiting without medical advice. According to Kew POWO, oleander is widely planted and naturalised and is therefore not globally threatened; the IUCN Red List currently treats it as Not Evaluated (NE). On the other hand, because it grows vigorously and is little browsed, it can become invasive in some areas beyond its native range, complicating urban green-space management.