This allows the skin to return to normal temperature between treatments. Generally speaking, you should apply ice for 10 to 30 minutes, depending on the body part and your own comfort level. Cold gel packs that can be frozen and refrozen cool the skin faster than an ordinary ice bag and should only be applied for short periods. Cold therapy may not be recommended for people who are very sensitive to cold, those with blood vessels too near the skin, and diabetics and others who have problems with diminished blood flow. If you're one of those people, you should talk to your doctor about applying ice to an injury.
Heat can also relax muscles and relieve pain, but it makes swelling worse by stimulating blood flow.
That's why cold is best immediately following an injury and for at least 48 hours afterward, or until the swelling goes down.