Brief Answer:
allergy

Detailed Answer:
Hello,

welcome to the 'Ask a Doctor' service.

Sudden swelling on the lips is usually caused by allergic reactions. Any allergen may trigger them. Drugs, food, perfumes, etc are all potential triggers. If you don't have anything in your mouth or close to it that might have caused swelling the an allergic reaction (including angioedema) is the most likely scenario.

Allergic reactions are treated with antihistamines first. If the upper airways are involved visiting the ER immediately is recommended. Upper airway involvement may present with shortness of breath, stridor, swelling of the structures inside the mouth (like the tongue or the cheeks), hoarseness, etc. When the airways are involved the situation becomes dangerous and potentially life-threatening as the swelling may get worse and cause suffocation.

When the swelling involves the mouth and surrounding tissues, angioedema is suspected. This is something like an allergic reaction that requires the same treatment. Angioedema may involve the airways so any of the symptoms I've mentioned above should alarm you to go to the ER urgently.

Since the lips are involved you'd better visit a doctor anyway for proper assessment. Other potential causes usually include local infections (like herpes) or injuries.

I hope I've answered your question. Please let me know if you need further assistance.

Kind Regards!

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