I had head injury two days ago and doctors suggest me to take eptoin 100 mg thrice a day while my cut scan normal. how long i need it.

Talk to your doctor: You probably mean "phenytoin" 100 mgs 3 times a day. This drug, which protects from having seizures after trauma or hemorrhage is commonly used for a few weeks until the patient is either "cleared" from having a serious injury or treated. The clearance process includes normal neurological evaluation, and normal MRI scans. You should discuss that with your doctor.