Jellyfish sting. Red rash, caused by a jellyfish sting, on a man's upper leg. Jellyfish are marine organisms whose tentacles contain stinging capsules that discharge when touched. The severity of the sting depends upon the species which inflicted it. Usually, the result of a sting is an itchy or mildly painful rash. In rare cases, venom in the sting can cause vomiting, sweating, shock, breathing difficulties and fainting. Treatment includes applying vinegar to the rash to inactivate the stinging capsules, and removing remaining tentacle fragments with adhesive tape. Painkilling drugs and antivenoms may also be used.