Question
I didn't see an answer to the question about the honey locust thorns being poison.  I just got jabbed with a honey locust thorn and it is swelling quite a bit.

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.  

Penn State Robert Nuss, professor of ornamental horticulture.

"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."  

we can rule out toxicology here, as Honey Locusts show no signs of toxicity, and only the seeds of the Black Locust are dangerous.
There is also the chance of an infection.

Now I guess that anything foreign to the body could be classified as a "poison" such as dirt or dust  on the thorn or the thorn itself but the plant itself is not toxic.  



Try soaking the area if you can,  in warm, soapy water for about 20-30 minutes, not only to clean it but also to soften the skin. Then, with a sterilized needle, tweezers, alcohol (not for the wound -- to disinfect the equipment and the skin before you explore) and a magnifying glass, see if you can find the entry channel, and follow it down. Don't go overboard and hack a gaping crater in your finger -- the ideal here is to be extremely gentle and also persistent. If you can find and remove the splinter, hit it with hydrogen peroxide, bandage, and watch it heal.

If you can't find the thorn, or can't get all of it out, an X-ray and a man with a scalpel (and a license to practice medicine) is called for.

The concern here is that, if the shard works its way into the joint, you may wind up having problems with that joint for some time to come, so  don't wait this one out too long.