Facial paralysis is the total loss of voluntary muscle movement of one side of the face.

Seventy-five per cent of cases of facial paralysis in adults are caused by an inflammation of the facial nerve called  Bell's palsy. This disorder (probably derived from a virus) is common, affecting 25 out of 100,000 individuals per year. It is more common in people with diabetes. It usually affects one side of the face (very rarely both sides at one time), causing a drooping mouth, drooling, and excessive tearing from one eye. Paralysis occurs within a few hours to a few days. There may also be a loss of taste on the front of the tongue on the affected side of the face. Sound may also seem louder on the affected side (hyperacusis).

While the facial distortion usually improves over time, there may be some permanent deformity in 20% of individuals. Full recovery is less likely in older people, as well as those with hyperacusis, loss of taste, and severe paralysis. Recurrence of paralysis on the same side or the opposite side occurs in 1 in 10 people.

In facial paralysis due to stroke, the eye on the affected side can be closed and the forehead can be wrinkled. This is not possible with Bell's palsy. Other muscles on one side of the body may also be involved with a stroke.

Facial paralysis due to a brain tumor generally develops gradually with accompanying headaches, seizures, or hearing loss.

In newborns, facial paralysis may result from birth trauma.