Heat therapy, such as that from a heating pad, or ice therapy, such as that from an ice pack, is often the first course of treatment people seek when they feel pain after taking pain medications. The application of heat or ice can often bring about much-needed relief, but only if you know which one to use for your type of injury.
WebMD explains that you would use heat to boost blood flow, while ice will restrict blood flow.
Use heat
You will want to use heat when you need to get more blood flowing to an area to promote healing or to warm up the muscles. It is best for long-term injuries, such as muscle stiffness. It can work great to loosen muscles that tighten up or for muscles that are cramping.
Use ice
Ice is best for fresh injuries. It helps to reduce swelling and can dull pain sensors. You would want to use ice immediately following an accident to help soothe bumps and bruises or to alleviate pain from sprains. You can use it on chronic conditions, though, if it is what brings you the best relief. Just be careful about using it too much, which can damage skin and cause healing issues.
Use either
In some situations, you can use either heat or ice if you want. Both can help ease the pain from a stiff neck or deal with back pain. They can also work well for short-term pain relief in a pinch, such as if you need something quick but are away from home or cannot get access to anything but ice.