A friend of me has got a cyst under his left armpit since he was 4years.he asked a doctor&told him that its a sebaceous cyst.could this cause danger?

Cyst Complications: Please see "Epidermoid cysts (sebaceous cysts)" by Mayo Clinic at: http://www.mayoclinic.org/diseases-conditions/sebaceous-cysts/basics/complications/con-20031599 . Complications may include: infection,inflammation, rupture. Skin cancer would be very rare.

Low risk: Sebaceous cysts are low risk for problems. They can become infected. Typically they are removed with a small surgical procedure, sometimes in the office. Be well.