The famous line uttered by Joe from Friends echoes down through jelly lore:
But is that really all it takes? Does urine really help with a jellyfish sting? From first hand experience I can tell you pee don’t do much, which is why I was thrilled to find a paper all about pee, meat tenderizer and other urban legends of the jellyfish sting world.
Jellyfish contain thousands of stinging cells on their tentacles, termed nematocytes (or cnidocytes). These cells each fire a long barb on contact, which injects venome into skin like a hypodermic needle. And believe me, these things are no joke:
But during a jelly sting, many of these cells get stuck on human skin unfired. According to jelly sting rumors, sting treatments like vinegar, urin and meat tenderizer break up these cells before they have a chance to inject their toxin.
In this study [1] two brave scientists willingly exposed themselves to sea wasps (Chiropsalmus quadrumanus) and Atlantic stinging nettles (Chrysaora quinquecirrha). For 20 days straight each authors placed a tentacle on their inner arm, followed by a particular treatment, and then rated the pain. Surprisingly, many of the most common treatments, like vinegar, alcohol and ammonia actually made the pain * worse*, not better. Pee, distilled water and meat tenderizer had no effect (as an aside: how awesome is it that these authors stung themselves and doused the sting with pee ALL FOR SCIENCE!). So, what did work? Pain killers. Doh. In this case, regular o’ lidocaine really is the way to go. But lidocaine doesn’t just make the pain go away, it actually prevents the undischarged stinging cells from firing. Other methods like vinegar and meat tenderizer can then be used to remove the cells.
And as for Monica, how did she fare? I choose to believe they’re all happy and subdued after a healthy dose of lidocaine and a rescued day at the beach. (or is that perhaps shame after reading this post?)
Work cited
[1] Evaluation of the effects of various chemicals on discharge of and pain caused by jellyfish nematocysts
http://www.sciencedirect.com/science/article/pii/S1532045610000098#
The Helm Lab Blog 
Vinegar is known to inhibit the firing mechanism of jellyfish stinging cells. Vinegar has never been used for pain relief, it’s prime function is to stop the injection of venom. Vinegar (containing > 5% acetic acid) is recommended as treatment for potentially fatal jellyfish species such as box jellyfish by the American Heart Association, the American Red Cross and the Australian Resuscitation Council. A recent preliminary study at James Cook University, Australia demonstrated that while vinegar neutralises undischarged stinging cells, it may cause further discharge of already discharged stinging cells. This has yet to be fully proven.