Will vineager help a jellyfish sting?

It doesn't.

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Vinegar will not lessen the effects of a sting. What it can do, in certain types of jellyfish, is deactivate any unfired stingers. Studies have clearly demonstrated that dousing box jellyfish tentacles for 30 seconds with household vinegar totally inhibits the firing of any undischarged stinging cells. This can certainly be a life-saving part of first aid treatment for box jelly stings, since any tentacles remaining on the victim would otherwise continue to fire more venom, especially when an attempt is made to remove any tentacle remnants from the skin.

On the other hand, the same studies also show that vinegar will cause the tentacles of a few other jellyfish species studied (including Man of War) to discharge some or all of their remaining stinging cells. Do not use vinegar for Physalia stings. Fresh water, urine and alcohol have also clearly been shown to trigger further stinger discharge, so for non-box type jellyfish stings, it's best to try to rinse tentacles off with sea water, scrape them off, or pluck them off. Definitely do not rub them or press on them.