Heat or ice for an injury? The evidence is thinner than the ritual suggests

"Ice a fresh injury, heat an old ache" is one of the most repeated pieces of household medicine, and it is held far more confidently than the evidence warrants. When you go looking for the trials behind it, they are small, few and mostly inconclusive. That does not make heat or ice useless — both are cheap, safe and can feel good — but it does mean the choice matters less than the ritual implies.

Ice on a fresh sprain: almost nothing to go on

The classic case for ice is the acute ankle sprain. A 2021 systematic review searched six databases for randomised or quasi-randomised trials of cryotherapy for acute ankle sprain and found just two, both at high risk of bias [s1]. Both tested ice added on top of another treatment rather than against nothing. The pooled picture was that cryotherapy did not enhance the effect of the other intervention on swelling (mean difference 6.0; 95% confidence interval 0.5 to 12.5), on pain intensity (mean difference -0.03), or on range of motion [s1]. The review's blunt conclusion was that the current literature lacks evidence supporting the use of cryotherapy in the management of acute ankle sprain, and it issued "an urgent call" for larger high-quality trials [s1].

That is not proof ice does nothing; it is the absence of good proof either way. Ice reliably numbs pain in the moment, which is a real benefit even if it does not speed healing. The old RICE formula (rest, ice, compression, elevation) was never built on strong trial data, which is why sports medicine has drifted toward gentle early movement rather than prolonged icing and rest.

Ice for sore muscles after exercise: a modest, real effect

The one place cold immersion has more behind it is delayed muscle soreness after hard exercise. A Cochrane review pooled 17 small trials involving 366 participants comparing cold-water immersion with doing nothing [s2]. It found statistically significant reductions in muscle soreness in favour of cold-water immersion at 24 hours (standardised mean difference -0.55; 95% CI -0.84 to -0.27), 48 hours (-0.66; -0.97 to -0.35), 72 hours (-0.93; -1.36 to -0.51) and 96 hours (-0.58; -1.00 to -0.16) [s2]. Those are small-to-moderate effects, the results were heterogeneous, the study quality was low, and most trials failed to actively monitor for harms [s2]. So an ice bath after a punishing session probably takes the edge off soreness the next day — but this is comfort, not a training accelerant, and pooling against contrast baths showed no clear winner [s2].

Heat for a bad back: the best-supported of the three

Heat, meanwhile, has its strongest evidence for low-back pain. A Cochrane review of superficial heat or cold included nine trials with 1117 participants [s3]. Across two trials of 258 people with a mix of acute and sub-acute low-back pain, a heat wrap significantly reduced pain after five days (weighted mean difference 1.06 on a 0-to-5 scale; 95% CI 0.68 to 1.45) compared with an oral placebo [s3]. One trial of 90 people found a heated blanket decreased acute low-back pain immediately after application (weighted mean difference -32.20 on a 0-to-100 scale; 95% CI -38.69 to -25.71) [s3], and adding exercise to the heat wrap reduced pain further after seven days [s3].

The mirror image is the striking part. The same review found there was insufficient evidence to evaluate the effects of cold for low-back pain, with only three poor-quality studies located, and conflicting evidence on whether heat or cold is better [s3]. The reflex to reach for an ice pack for a sore back has essentially no trial support behind it.

The honest position

The rule of thumb survives because heat and ice are harmless, available and soothing, and because almost everything they are used for improves on its own. The trials say: for a sore back, a heat wrap is the best-supported choice [s3]; for muscle soreness after exercise, cold immersion gives a modest, short-lived edge [s2]; and for a fresh sprain, there is barely any evidence that ice changes the outcome at all, though it will dull the pain [s1]. Use whichever feels better, keep the skin protected, and do not expect either one to speed the actual repair.

This article is informational and is not medical advice.

Sources

• [s1] Effectiveness of cryotherapy on pain intensity, swelling, range of motion, function and recurrence in acute ankle sprain — Physical Therapy in Sport, May 2021. https://doi.org/10.1016/j.ptsp.2021.03.011
• [s2] Cold-water immersion (cryotherapy) for preventing and treating muscle soreness after exercise — Cochrane Database of Systematic Reviews, 15 February 2012. https://doi.org/10.1002/14651858.CD008262.pub2
• [s3] Superficial heat or cold for low back pain — Cochrane Database of Systematic Reviews, 25 January 2006. https://doi.org/10.1002/14651858.CD004750.pub2