What Causes Grey Hair?

i'm having white beard more in age 29 i got fix 2 years back so that i'm eating eptain tablets daily one in night the doctor said due to late nighy slpeeing, over thinking this is cause of fix i got in medical report. now i goy white beard more but in small age i had more tablets taken like every body when age 17 i got white hair in head now i got white beard, some said to drink amla juice twice a day so i'm following that