Fits (seizures) have a variety of causes, and most fits will not be due to a brain tumour.
Fits occurring for the first time should be viewed as a potentially serious symptom, and require further investigation of the cause.
A fit can be a brief moment when the person appears to be “absent” from what is going on around them, or jerking/twitching of a hand, arm or leg or jerking/twitching affecting the whole body. They may or may not become unconscious.
If you're a teenager and you experience a fit or seizure, always seek medical help. Fits can be due to a serious illness and any teenager having a first fit must be reviewed as soon as possible by a doctor.