The reaction that you have to a jellyfish or Portuguese man-of-war sting depends on many factors. The potency of the venom changes with the type of jellyfish and also is stronger during some seasons than in others. Other factors that affect the severity of your reaction include:

• Your size, age, and general health.
• The location and how much of your body (surface area) was stung.
• The size or number of jellyfish or tentacles that stung you.

The pain from jellyfish and Portuguese man-of-war stings may be intense and may last for several hours. The skin at the site of the stings may look dusky or bluish purple. Blisters and deep sores (ulcers) may develop. Healing is generally a slow process that may take many weeks. Permanent scars may occur at the site of a sting.

Small children are at risk for stings in the mouth if they put a jellyfish or portion of a tentacle in their mouths. A sting to the mouth or throat can cause sudden and severe swelling, which can be life-threatening.

Problems from jellyfish or Portuguese man-of-war stings may develop immediately or be delayed for several hours or days. A severe allergic reaction (anaphylaxis) may affect any body system and require emergency care. Symptoms can include any of the following:

• Difficulty breathing or wheezing
• Severe pain
• Sudden major swelling that increases rapidly
• Nausea, vomiting, or diarrhea
• Fever and chills
• Headache, dizziness, or vertigo
• Muscle spasm, muscle cramps, joint pain, or generalized aching