When to Use Heat or Ice on an Injury

A rule of thumb covers most situations. Use ice for a new injury that is swollen, hot or bruised, usually in the first two or three days. Use heat for stiffness, tight muscles and aches that have been around for a while, once any swelling has gone down. Ice calms things down; heat loosens things up.

The rest of this guide explains why, how to apply each without harming your skin, what to do when you cannot tell which you need, and when a sore spot should be seen rather than treated at home.

Why cold and warmth have opposite effects

After a knock, sprain or strain, the body sends extra blood and fluid to the area. That is part of healing, but it also brings swelling, heat and throbbing. Cold narrows the small blood vessels near the skin and slows nerve signals, which can limit swelling and dull pain for a while.

Heat does the reverse. It widens blood vessels, increases blood flow and helps muscles and connective tissue relax. That is welcome when a muscle is tight or a joint is stiff, but unhelpful when an area is already inflamed, because more blood can mean more swelling.

Heat or ice at a glance

How to use ice safely

1. Wrap it. Never put ice, an ice pack or frozen peas straight on bare skin. A thin, damp tea towel works well.
2. Keep sessions short. Around 15 to 20 minutes at a time is the usual guide, repeated every two to three hours while you are awake during the first few days.
3. Check the skin. Some redness is normal. If the skin turns white or blotchy, feels numb or burns, take the pack off.
4. Combine with rest and elevation. Raising a swollen ankle or knee above the level of your heart while you ice it helps fluid drain away.

How to use heat safely

• Choose gentle warmth. A hot water bottle filled with hot rather than boiling water, a microwavable wheat bag or a warm shower all work. It should feel comfortably warm, never hot.
• Use a layer. Wrap hot water bottles and wheat bags in a cover or towel.
• Limit the time. Twenty minutes or so is plenty for most aches. Don't fall asleep on a heat pad or electric blanket.
• Follow with movement. Warmth is a good lead-in to stretching or the kind of light exercises in our physical therapy band starter set, as long as they don't make the pain worse.

Who should take extra care

Both cold and heat rely on you feeling when the skin has had enough. Be cautious, and ask a GP, pharmacist or physiotherapist first, if you:

• have diabetes, nerve damage or any condition that reduces feeling in the skin
• have poor circulation, including Raynaud's phenomenon
• are applying it over a wound, a recent scar or an area of broken or fragile skin
• have a condition affecting your heart or blood pressure
• are caring for a young child or an older person who may not be able to say when it is too hot or too cold

Don't use heat on a part of the body that is already hot, red and swollen, or where an infection is possible.

Aches after a car accident

Neck and back pain after a collision often appears a day or more later, when stiffness sets in. In the first days a cold pack can ease soreness; after that, many people find warmth and gentle movement more helpful. The guide on treating whiplash injuries covers the self-care that sits alongside this, and aches and pains after a car accident lists the symptoms that should be checked before you treat anything at home.

Common questions

Can I alternate heat and ice?

Some people switch between them for stubborn muscle aches once swelling has gone, and find it comfortable. In the early, swollen phase of an injury, sticking with cold is the safer choice.

What if I can't tell which one I need?

If the area is swollen, warm to the touch or the injury happened in the last couple of days, start with ice. If it is stiff, tight and has been that way for a while without swelling, try heat. If one makes it feel worse, stop and try the other, or neither.

Are there other drug-free options?

Yes. Rest, gentle movement, supportive positions for sleep and devices such as a TENS unit all help some people. None of them replaces having an injury looked at when it needs it.

When to get checked instead

Heat and ice ease symptoms; they do not diagnose anything. Get medical advice if you cannot put weight on a leg or use an arm, if a joint looks out of shape, if there is numbness or tingling that does not settle, if swelling or pain keeps getting worse after a few days, or if pain follows a high-speed accident or a heavy fall. For severe pain, a suspected broken bone or any injury to the head, seek urgent help.