Is it dangerous if i use medicines for flu without doctor's prescription? i often use my parents'

That depends on what: You are taking. Looks like you are 19. Taking antibiotics that is dangerous. If you take tylenol, (acetaminophen) nsaid's, Benadryl or other otc medications as directed for each med that may be alright. If you are taking your parent's prescription medications that isn't ok. If using over the counter medications & aren't sure about how to use them - talk to your parents, your pharmacist or physician.