There are three goals of first aid for uncomplicated jellyfish stings: prevent injury to rescuers, inactivate the nematocysts (stinging cells), and remove any tentacles stuck on the patient. To prevent injury to rescuers, barrier clothing should be worn. Inactivating the nematocysts, or stinging cells, prevents further injection of venom into the patient.
Vinegar (3 to 10 % acetic acid in water) should be applied for box jellyfish stings. However, vinegar is not recommended for Portuguese Man o' War stings. In the case of stings on or around the eyes, vinegar may be placed on a towel and dabbed around the eyes, but not in them. Salt water (or fresh water if sea water is unavailable) may also be used in case vinegar is not readily available. Rubbing the wound, or using alcohol, spirits, ammonia, or urine will encourage the release of venom and should be avoided.
Once deactivated, the stinging cells must be removed. This can be accomplished by picking off tentacles left on the body. After large pieces of the jellyfish are removed, shaving cream may be applied to the area and a knife edge, safety razor, or credit card may be used to take away any remaining nematocysts.
Beyond initial first aid, antihistamines such as diphenhydramine (Benadryl) may be used to control skin irritation.