How long to keep an onion under the armpit for a fever and how many days does last?

Do not put an onion under your armpit for a fever.

This is not a medically recommended treatment and has no scientific evidence to support its effectiveness. Additionally, it may pose a risk of infection or other health complications.

Recommended methods to reduce a fever include:

- Taking over-the-counter fever reducers such as paracetamol or ibuprofen.

- Drinking plenty of fluids.

- Resting.

- Applying a cold compress to the forehead.

- Taking a lukewarm bath.

If the fever persists or is accompanied by other concerning symptoms, consult a healthcare professional.