I encounter this question often from patients in the office.
Ice is commonly used for acute injuries – meaning that you have just twisted your ankle, sprained your back, etc. Ice provides the most benefit in the first 24-48 hours after an injury. The ice minimizes swelling and inflammation, which in turn decreases pain.
Typically, ice is applied to the area of pain for 20 minutes. More than this can cause cold injury to the tissues, and in extreme cases frostbite. After the 48 hour mark, discontinue the use of ice.
Heating pads are most beneficial for chronic pains – like recurrent back pain, or known shoulder problem. Heat will relax and loosen tissues by increasing blood flow to the area of pain.
Heating pads can be applied to the area of pain for no longer than 20 minutes at a time. Skin burns have occurred with longer heating times. Specifically, do not go to sleep with a heating pad applied anywhere.