What Causes Bell’s Palsy? Why the Face Suddenly Weakens and Who Is Most at Risk

Many people wake up to find that one side of their face won’t move — with no warning at all. The first question is always: why did this happen? This guide explains what happens to the facial nerve, the known causes of Bell’s palsy, and who is most at risk.

What is the facial nerve and where does it run?

The facial nerve (cranial nerve VII) is one of 12 pairs of cranial nerves. It leaves the brainstem, passes through a very narrow bony canal in the temporal bone behind the ear, then branches across the face to control:

• The forehead muscles and eyebrow raising.
• Eyelid closure and blinking.
• Smiling and movement of the lips and cheek.
• It also carries taste from the front of the tongue and helps control tears, saliva and the ear’s sensitivity to sound.

Because the canal is so tight, any swelling of the nerve compresses it, weakening or blocking its signals — and one side of the face becomes paralysed.

The most likely cause: viral inflammation

Most cases are called Bell’s palsy, meaning facial palsy of unknown (idiopathic) cause. Research suggests that:

1. A virus that lies dormant in the body — most often herpes simplex virus type 1 (HSV-1), which causes cold sores — reactivates.
2. The facial nerve becomes inflamed and swollen.
3. The swelling squeezes the nerve inside its bony canal, cutting off signals to the facial muscles.

This explains why symptoms start suddenly and peak within 48–72 hours, why most people recover as the inflammation settles, and why early corticosteroids are given — they reduce the swelling. See our Bell’s palsy treatment guide.

Risk factors

Bell’s palsy can affect anyone at any age, but it is most common between 15 and 60, and the risk rises with:

Risk factor	Why it matters
Diabetes	Affects small nerves and circulation and may slow recovery
Pregnancy, especially the third trimester and just after birth	Immune changes and fluid retention — read facial palsy in pregnancy
Colds and flu	Upper respiratory infections often come a few days before
High blood pressure and obesity	Linked to higher risk in several studies
Previous episode or family history	A minority of cases recur — read recurrent Bell’s palsy

Other causes your doctor must rule out

Not every facial palsy is Bell’s palsy. Your doctor will look for causes that need different treatment:

• Ramsay Hunt syndrome — caused by the chickenpox virus, with a painful rash in the ear. Read our Ramsay Hunt syndrome guide.
• Stroke — usually weakens the lower face while the forehead still moves, and may come with arm or speech problems. See Bell’s palsy vs stroke.
• Severe middle ear infections.
• Lyme disease in areas where ticks are common.
• Head or facial injury, or surgery near the ear.
• Tumours pressing on the nerve — rare, and suspected when weakness develops gradually over weeks or never improves.
• Rarer conditions such as Guillain–Barré syndrome and sarcoidosis.

Call emergency services immediately if you also have arm or leg weakness, slurred speech, severe dizziness, or weakness on both sides of the face.

Can Bell’s palsy be prevented?

There’s no guaranteed way to prevent it, but you can reduce general risk factors:

• Keep blood sugar and blood pressure under control.
• Get enough sleep and manage chronic stress to support immunity.
• Treat colds and ear infections properly.
• Ask your doctor about the shingles (zoster) vaccine, especially after 50.

What to do now

Knowing the cause helps, but what matters most is what you do in the first days:

1. See a doctor within 72 hours to start the right treatment.
2. Protect your eye if it doesn’t close fully — see our eye care guide.
3. Start guided facial exercises once your doctor agrees.

Raq supports you at this stage with a structured 3-month rehab plan and real-time AI guidance through your camera during every exercise, so each movement is done correctly. For the full picture, read the complete guide to Bell’s palsy.