Question
My friend was pierced by a lotus tree thorn. Is there poison in the thorn?

Answer
There is not really a poison in the thorn but being stuck by any number of thorns (rose. locust etc.)they all seem to cause a medical problem. More then likely the dirt and dust gets into the wound and causes infection. Or when it is pulled out a small bit remains and festers.
The only site I found was from Penn State Robert Nuss, professor of ornamental horticulture.
http://www.aginfo.psu.edu/News/october99/toxic.html

"Thorny plants. The most common plant-related injuries are caused by thorns. "Most people don't seek medical help after getting stuck by a thorn," Nuss says. "But implanted thorns, needles or spines can cause infections or other medical problems."

Nuss says thorns embedded near joints can cause chronic arthritis. Thorns embedded near bones can mimic a bone tumor. Thorny plants that can cause painful injuries include roses, black locust and honeylocust trees and blackberry bushes."