What does it mean if your pupils are dilated?

Pupil size naturally gets larger in low light. Other reasons for both pupils to dilate in a conscious person include medication side effects; use of illegal drugs that act as a stimulant; or extreme excitement, fear or agitation. Dilation of both pupils or only one pupil in someone who has had a head injury or has any other neurological symptoms should prompt immediate medical attention, because it could be a sign of brain damage.