The Short Answer
Multiday running sends almost everyone to the medical tent at some point, but almost always for something small. The best stage-race data show 85% of runners needing care and about 95% of those problems being minor. The serious problems are kidney injury, low blood sodium, falls, cold, and sudden cardiac death. They are uncommon, and some of the runners at highest risk can be identified before the start. This page covers how often each problem occurs, what kills runners, who should see a doctor first, and the warning signs that mean stop.
How Often Things Go Wrong
| Problem | How often | Where the number comes from |
|---|---|---|
| Any medical care | 85% of runners; 3.86 encounters per runner; about 95% minor | 396 runners in 7-day, 250-km desert stage races (Krabak 2011) |
| Skin and blisters | 74.3% of all medical encounters | Same stage-race study; skin problems were most likely on days 3 and 4 |
| Muscles, tendons, and bones | 18.2% of encounters | Same study; also most likely on days 3 and 4 |
| Medical illness | 7.5% of encounters | Same study; most likely on day 1 |
| Gut symptoms | 96% of runners; nausea in 60.3%; 35.6% of non-finishers named gut symptoms as a reason for dropping | 272 runners at the 161-km Western States Endurance Run (Stuempfle 2015) |
| Low blood sodium (hyponatremia) | 15.1% of finishers; 4.6% to 51% depending on the year, higher in hot years | 669 finishers tested over five years of 161-km races in northern California (Hoffman 2013) |
| Acute kidney injury | 41.4% cumulative incidence, defined by blood creatinine reaching at least 1.5 times baseline | 128 runners in 6-stage, 250-km desert races; women had 4.6 times the odds (Lipman 2016) |
| Sleep-deprivation symptoms | 80% of finishers, including falls and hallucinations | 1,154 finishers of 111-km and 165-km mountain ultras (Kishi 2024) |
Two patterns stand out. Age and experience protect: in the stage-race study each extra decade of age meant 0.5 fewer injuries or illnesses, and in the 1,212-runner ULTRA Study the injured runners were younger and less experienced than the uninjured ones. And the sodium and kidney figures come from blood drawn at checkpoints or the finish, so they count laboratory-defined cases, not hospital admissions.
Sleep loss is the risk that sets multiday racing apart. In Kishi and colleagues' survey, four in five finishers had symptoms they attributed to sleep deprivation, with falls and hallucinations among them, and 22% believed it raised their risk of accidents in daily life after the race. A nap plan is safety equipment; the sleep strategy guide shows how to build one for each format.
Deaths and Cardiac Arrest
Deaths are rare enough that no one has calculated an ultra-specific death rate or cardiac-arrest rate. The best count comes from Roi's review of mountain running in Western Europe: 51 deaths linked to races and race training from 2008 to 2019. Of those, 43% were sudden cardiac deaths, 32% were falls, and 16% were hypothermia; the rest were lightning strikes, a combination of fall and cold, and one deer attack. Most (69%) happened during races. Forty-five of the 51 were men, with an average age of 50.
For cardiac arrest, the nearest benchmark is the road. Among 29.3 million US marathon and half-marathon finishers from 2010 to 2023, Kim and colleagues counted 0.60 cardiac arrests per 100,000 runners, and 1.04 per 100,000 in the marathon alone. The share of arrests that ended in death fell from 71% in 2000–2009 to 34%, which the authors linked to fast CPR and defibrillator access. These are marathon and half-marathon figures, not ultra figures, and nobody knows whether the ultra rate is higher or lower: ultras are run at lower intensity, but they last far longer and rescue in remote terrain is slower.
Who Should See a Doctor First
Race cardiac deaths mostly come from coronary artery disease in middle-aged men. Coronary artery disease was the most common cause among the marathon arrests with a known cause, men had about six times the arrest rate of women, and the men who died in the mountain series averaged 50. Our recommendation is a sports-medicine or cardiology review before your first multiday race if any of these apply:
- Chest pain, pressure, or unusual breathlessness during exercise
- Fainting or near-fainting during or just after exercise
- Palpitations, or a known heart rhythm problem
- Known heart disease, or a close relative who died suddenly at a young age
- Age 40 or older with coronary risk factors such as smoking, diabetes, high blood pressure, or high cholesterol
- Kidney disease, or a past episode of exertional rhabdomyolysis, heat stroke, or exercise-associated hyponatremia
- Medicines that affect blood pressure, fluid balance, or the kidneys
This list is our editorial synthesis of the studies above, not a formal screening protocol. A clean check-up does not cover symptoms that start later; new symptoms in training are a reason to go back.
How to Lower the Risk
- Get screened if your history calls for it. Use the list above, and book the appointment months before the race, not the week before.
- Build load gradually. Inexperience is the clearest injury risk factor in the data. The 24-hour race training guide shows how to build volume, back-to-back days, and time on feet.
- Test your fueling in training. More than a third of Western States non-finishers named gut symptoms as a reason for dropping. Practice race food at race pace on long runs; the gut training guide has a progression.
- Drink to thirst. The international hyponatremia consensus recommends thirst-guided drinking, and the authors of the 161-km sodium study called drinking beyond thirst ill advised. Weight gain during a race means you are overdrinking, and extra sodium does not make overdrinking safe. The hydration and sodium guide has the fluid and sodium numbers.
- Take no NSAIDs during the race. In a randomized trial over 80 km in desert races, 52% of runners on ibuprofen developed acute kidney injury against 34% on placebo. The trial was small and the gap was not statistically conclusive, but it works out to one extra case for every 5.5 runners who took the drug. Up to 75% of ultrarunners take NSAIDs while racing.
- Have a sleep plan. Decide in advance when and how long you will sleep, and treat weaving, stumbling, or hallucinating as the signal to nap, not to push on.
- Brief your crew and find the medical tent. Give your crew the crew instructions template and the red-flag list below, and agree in advance that they can stop you.
Medical Red Flags: When to Stop
These signs are not normal race fatigue. If you or your crew notice any of them, stop and get help. Do not try to tell dehydration, low blood sodium, rhabdomyolysis, or a heart problem apart from symptoms alone.
Heart, breathing and collapse
- Chest pain, pressure or tightness
- Fainting, near-fainting or collapse
- Severe shortness of breath, or a racing or irregular heartbeat that does not settle with rest
Brain and behavior
- New confusion, disorientation, slurred speech or a sudden behavior change
- Hallucinations, or falling asleep while moving
- Declining responsiveness, or a seizure
- Repeated falls, stumbling or loss of coordination
- Severe or worsening headache, especially with nausea or vomiting
Fluids, kidneys and muscles
- Repeated vomiting, or unable to keep fluids down
- Puffy hands or face with weight gain, headache or nausea (a possible sign of overdrinking and low blood sodium)
- Tea- or cola-colored urine, or little or no urine, especially with marked muscle pain or weakness
Temperature and injury
- Unable to keep warm, uncontrollable shivering, or hot and confused
- Rapidly worsening illness, or an injury that changes the runner's gait
- Spreading redness, warmth, pus or fever around a wound or blister
What About Long-Term Health?
A 2022 review by Scheer and colleagues found that serious acute events are rare but that evidence of long-term problems is mounting, clearest for the heart (right-ventricular dysfunction and a possible higher risk of arrhythmia and high blood pressure) and, in rare cases, kidney injury that may progress to chronic kidney disease; our answer to is ultrarunning bad for your body? covers that evidence system by system.
Sources
- Krabak, B.J. et al. (2011) — Study of injury and illness rates in multiday ultramarathon runners. Medicine & Science in Sports & Exercise, 43(12), 2314–2320.
- Stuempfle, K.J. & Hoffman, M.D. (2015) — Gastrointestinal distress is common during a 161-km ultramarathon. Journal of Sports Sciences, 33(17), 1814–1821.
- Hoffman, M.D. et al. (2013) — Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Medicine & Science in Sports & Exercise, 45(4), 784–791.
- Lipman, G.S. et al. (2016) — Incidence and Prevalence of Acute Kidney Injury During Multistage Ultramarathons. Clinical Journal of Sport Medicine, 26(4), 314–319.
- Kishi, A. et al. (2024) — Sleep and Ultramarathon: Exploring Patterns, Strategies, and Repercussions of 1,154 Mountain Ultramarathons Finishers. Sports Medicine - Open, 10(1), 34.
- Hoffman, M.D. & Krishnan, E. (2014) — Health and exercise-related medical issues among 1,212 ultramarathon runners: baseline findings from the Ultrarunners Longitudinal TRAcking (ULTRA) Study. PLOS ONE, 9(1), e83867.
- Roi, G.S. (2021) — Fatal Events Related to Running Competitions in the Mountains. Wilderness & Environmental Medicine, 32(2), 176–180.
- Kim, J.H. et al. (2025) — Cardiac Arrest During Long-Distance Running Races. JAMA, 333(19), 1699–1707.
- Hew-Butler, T. et al. (2015) — Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303–320.
- Lipman, G.S. et al. (2017) — Ibuprofen versus placebo effect on acute kidney injury in ultramarathons: a randomised controlled trial. Emergency Medicine Journal, 34(10), 637–642.
- Scheer, V. et al. (2022) — Potential Long-Term Health Problems Associated with Ultra-Endurance Running: A Narrative Review. Sports Medicine, 52(4), 725–740.