I had sebacious cyst on my back which ruptured on its own and the skin is flat now.. what next? how to ensure that sac is removed? any medication?

Consider Excision: If you had a ruptured sebaceous cyst that is now healed and shows no signs of infection, such as redness or tenderness, then you do not need any medication at this time. At this point, you may want to wait to see if the cyst recurs and, if it does, have it removed. Your other option is to have the area where the cyst was removed now, in effort to prevent future recurrence.