Why NSAIDs Shouldn't Be Used During Ultra-Distance Running
NSAIDs (non-steroidal anti-inflammatory drugs) such as ibuprofen, naproxen, and diclofenac are commonly used to relieve everyday aches and pains. However, during ultra-distance running they can significantly increase the risk of acute kidney injury (AKI), as well as contribute to salt imbalance and gastrointestinal problems.
AKI is surprisingly common in ultramarathons, affecting an estimated 30–80% of runners, depending on factors such as race distance, environmental conditions, heat, altitude, and how AKI is defined. Most cases resolve within 48–72 hours, but repeated episodes may increase the risk of developing long-term kidney damage.
Here's a simple explanation of what's happening inside your body during an ultra—and why NSAIDs can make matters worse.
What Happens to Your Kidneys During an Ultra?
The Squeeze – Reduced Blood Flow
During prolonged exercise, your body prioritises blood flow to working muscles and the skin to support movement and cooling. As a result, the kidneys receive less blood than usual, reducing their ability to filter waste efficiently.
Think of it as your kidneys being squeezed because they're running on a reduced blood supply.
The Muscle Crumble – Rhabdomyolysis
Hours of sustained effort cause muscle fibres to break down. This releases myoglobin, a muscle protein that must be filtered by the kidneys. In large amounts, myoglobin can contribute to kidney injury, particularly when combined with dehydration or reduced kidney blood flow.
Your muscles create debris, and your kidneys have to clear away the mess.
The Muddy Aftermath – Cellular Debris
Following an ultra, urine microscopy may reveal "muddy brown casts"—tiny clumps of damaged kidney cells and proteins that are shed as the kidneys recover from stress.
It's a bit like sweeping up dust and rubble after a renovation.
Where NSAIDs Make Things Worse
They Reduce Kidney Blood Flow Even Further
NSAIDs block the production of prostaglandins—chemicals that help keep blood vessels supplying the kidneys open. During an ultra, when kidney blood flow is already reduced, this can increase the risk of acute kidney injury.
They May Increase the Risk of Exercise-Associated Hyponatraemia
NSAIDs can alter kidney handling of water and may enhance the effects of antidiuretic hormone (ADH), making it harder to excrete excess water. Combined with heavy sweating and excessive drinking, this may increase the risk of exercise-associated hyponatraemia (EAH)—dangerously low blood sodium.
They Increase the Risk of Gut Problems
During prolonged exercise, blood flow to the stomach and intestines falls. NSAIDs reduce the protective lining of the gastrointestinal tract, increasing the risk of stomach irritation, ulcers, bleeding, and gastrointestinal symptoms.
They Can Mask Important Warning Signs
Pain is your body's way of signalling that something is wrong. NSAIDs may hide symptoms of injury, dehydration, or excessive strain, making it easier to push beyond safe limits.
How to Reduce the Risk of AKI During Ultras
Avoid taking NSAIDs during an ultra-distance event unless specifically advised by a healthcare professional.
Drink to thirst rather than drinking continuously. This helps reduce the risks of both dehydration and exercise-associated hyponatraemia.
Recover properly between races. Although AKI often resolves within a few days, repeated kidney stress may contribute to the development of chronic kidney disease over time.
Pay attention to warning signs such as severe muscle pain, persistent vomiting, confusion, swelling, reduced urine output, or cola-coloured urine, and seek medical assessment if they occur.
The Bottom Line
Ultra-distance running already places considerable stress on your kidneys. Adding NSAIDs at the same time increases that strain by reducing kidney blood flow, increasing the risk of gastrointestinal complications, and potentially contributing to dangerous fluid and electrolyte disturbances.
Avoiding NSAIDs during races is one of the simplest ways to protect your kidneys, reduce the risk of complications, and help keep you running healthy for years to come.
References
Lipman GS, et al. Ibuprofen versus placebo effect on acute kidney injury in ultramarathon runners. Emergency Medicine Journal. 2017.
Hodgson LE, et al. Acute kidney injury associated with endurance events—is it a cause for concern? BMJ Open Sport & Exercise Medicine. 2017.
Poussel M, et al. Acute Kidney Injury and Hyponatremia in Ultra-Trail Racing: A Systematic Review. IJERPH. 2022.
Crawley A, et al. Health effects of NSAID use in marathon and ultraendurance running: A scoping review. Sports Medicine Open. 2024.
Hew-Butler T, et al. Exercise-Associated Hyponatremia: 2017 Update.