Oleander is as beautiful as it is deadly. The plants contain nerioside and oleondroside, both of which are poisonous to humans and pets. Not only are the leaves, flowers and branches of oleander toxic; honey made from oleander nectar, water from its soil and smoke from its burned branches are all poisonous as well.
Should you ingest any oleander, you may experience: fatigue, rash, hives, vomiting, abdominal cramping, diarrhea, blurred vision, disorientation and headache. In rare cases (including chronic overdose), you can die.* Which begs the question: why is such a severely poisonous shrub planted so frequently along freeways in California?
Glad you asked! They are used in areas where humans are not likely to walk, such as the side of a freeway. Because Oleander are tall, full evergreen shrubs, they make good screens or natural fences in areas where privacy and noise pollution are a problem, such as the side of a freeway. Moreover, they require plenty of sun but not much water, making them ideal for areas where constant care isn't possible, such as--you guessed it--the side of the freeway! Their pretty flowers are just a bonus.
*If you're chomping on a branch right now, go call poison control. Also glory be to Yahoo! plants and health.