I have a large cost filled with puss on the back of my head, it drains sometimes and is very painful.

Might be infected: Best to have the area surgically treated. Normally, lancing the cyst and antibiotics are the first step in treatment. Once the infection has resolved, the cyst can be excised. Cysts are usually benign, but anything suspicious should be sent for pathologic analysis.

Infected Cysts: You need to be seen by a family doctor or a dermatologist to have this cyst drained and possibly excised. Do not wait. Go get this fixed.