No Urine on the jellyfish stings!
Don’t do it! Don’t pee on the jellyfish sting!
According to this Scientific American article not only will it not help, it will make it worse. So will fresh water. Your best bet is to rinse with salt water.
Basically osmosis. Remember science class and the idea of sleeping on the book so that the ideas would move into your head by osmosis? Yeah, I wish too…
It works like this, things tend to try to equalize. So if the inside of a cell has a lot of salt in it, and the water around it has none, the salt will move into the fresh water until it reaches a balance where the salt levels are even on both sides.
Now apply that idea to the jellyfish sting. Putting urine or fresh water on the tiny cnidocytes (stingers) left on your skin causes the venom to rush out of the cells to try to balance with the fluid. That means more stinging for you.
What about vinegar?
The above mentioned article seems to support it, but some other articles argue that in certain cases vinegar, just like urine, will make it worse.
The best thing seems to be a good rinse with salt water first and then perhaps isopropyl alcohol. But make sure you have removed anything still attached! Don’t leave a slimy jellyfish tentacle wrapped around your leg! Most things you do to it will release more venom. Get it off first, (NOT with bare fingers!) then start rinsing with the water right there at the beach.
Whatever you do, don’t urinate on it. It won’t help and you and your friends won’t respect each other in the morning.
If you ever end up in that situation, just think about how many shrinkage comments I just helped you avoid…