Sebaceous cyst...?

i had a strange kind of boil between my ...and often used to appear again after one or two months...its very painfull...it drains by itself..and vanishes and then again appear after one month...the doctor has said that it can be sebaceous cyst...now i am worried a bit...that is it a dangerous cyst...???can any body tel me that does it grow even bigger??its already very big..i am afraid of it....what should i do?