Boston Marathon 2026: The Medical Risks of Running 42 Kilometres — What Every Canadian Runner Needs to Know

The 130th Boston Marathon is underway today, April 20, 2026, with tens of thousands of runners tackling the iconic 42.2-kilometre course through the streets of Massachusetts. For the hundreds of Canadian runners among them — and for the hundreds of thousands more who dream of completing their first marathon — the race raises a question that doctors hear every spring: when does distance running become dangerous?

The 2026 Race: Conditions and Context

This year's Boston Marathon, presented by Bank of America, sees runners starting in cool temperatures in the low single digits Celsius, with Boston forecast to reach a high of around 11°C by mid-afternoon, according to the Boston Globe. Cool conditions generally reduce the risk of heat-related illness compared to warm-weather marathons — but cool weather introduces its own hazards, including muscle stiffness, hypothermia risk in slower finishers, and a tendency to underhydrate.

Boston EMS has activated additional ambulances citywide, and the Boston Public Health Commission's Stephen M. Lawlor Medical Intelligence Center is tracking all patients transported to hospital during the race. Medical stations are positioned along the course and at the finish line. Major races plan for emergencies not as an afterthought but as a core part of the event.

What the Research Actually Shows About Marathon Medical Risks

Running a marathon is, for most healthy adults, a safe and health-promoting activity. But the medical literature is clear that serious complications do occur, and certain populations face elevated risk.

Sudden cardiac arrest is the most feared outcome. A study published in NEJM Evidence found that cardiac arrest occurs in approximately 1 in 100,000 marathon finishers, with risk significantly higher in men over 45 with undiagnosed coronary artery disease. The key phrase is "undiagnosed" — many victims have no prior symptoms. Vigorous exercise temporarily increases cardiac risk, which is why pre-race medical screening is not just a bureaucratic hurdle but a genuine clinical tool.

Hyponatraemia (low blood sodium) is actually more common than cardiac events at mass-participation events. It occurs when runners drink excessive amounts of plain water without replacing electrolytes, diluting sodium in the blood to dangerous levels. Symptoms include nausea, disorientation, and seizures. Counterintuitively, the risk is highest in slower runners who spend more time on the course and drink steadily at every water station. Drinking to thirst, not on a schedule, remains the recommendation from sports medicine bodies.

Musculoskeletal injuries account for the vast majority of medical tent visits: knee pain, shin splints, stress fractures, blisters, and muscle cramping. Most are not life-threatening but can sideline a runner for weeks or months post-race if not properly managed.

Who Should See a Doctor Before Running a Marathon?

The Canadian Cardiovascular Society recommends that adults over 35 who have not had a recent medical evaluation — and anyone with symptoms of chest pain, unusual shortness of breath, palpitations, or dizziness during exercise — consult a physician before undertaking endurance training.

This is not about discouraging participation. It is about identifying the relatively small number of people for whom high-intensity endurance exercise poses a specific, manageable risk. An exercise stress test or ECG may be recommended depending on risk factors. The goal is to rule out conditions that could manifest catastrophically under physical stress.

Runners who have previously experienced:

• Fainting or near-fainting during or after exercise
• Chest pain with exertion
• A family history of sudden cardiac death in a first-degree relative under 50
• An unexplained drop in athletic performance

...should not dismiss these signs as "just training fatigue." These are reasons to consult a doctor, not warnings to ignore.

The Most Common Mistakes Post-Race (That Lead to Doctor Visits)

Finishing a marathon is not the end of the medical story. Sports medicine physicians in Canada see a predictable pattern of post-marathon presentations, particularly in amateur runners who did not train adequately or who raced too aggressively relative to their fitness level.

Stopping abruptly at the finish line causes blood to pool in the legs, reducing return to the heart and potentially triggering fainting. Walking for 10 to 15 minutes after crossing the finish line is standard advice — and often ignored.

Ignoring rhabdomyolysis symptoms (extreme muscle breakdown resulting in dark-coloured urine) is a serious post-race risk in undertrained runners. It can cause kidney damage if not treated. Any runner who notices dark or cola-coloured urine after a race should seek emergency care.

Returning to training too soon is among the most common reasons runners develop chronic injuries. Most sports medicine specialists recommend a minimum of two to four weeks of rest after completing a full marathon before resuming structured running — and more if pain or swelling persists.

What Canadian Runners Should Know Before Race Day

Health Canada does not maintain a national registry of marathon-related medical events, but provincial health authorities receive runners in emergency departments every spring marathon season. For Canadian runners training for their first major event — Toronto Waterfront, Vancouver Marathon, Ottawa Race Weekend — the key preparation steps are:

1. Complete a pre-race medical evaluation at least 8 weeks before race day, particularly if over 40, male, or sedentary prior to training
2. Understand your warning signs — do not push through cardiac symptoms during a race
3. Train the hydration strategy, not just the distance — practice your fuelling and fluid intake on long training runs
4. Have a recovery plan — arrange follow-up with your family doctor if you experience unusual symptoms in the two weeks post-race

According to the Government of Canada's Physical Activity Guidelines, most Canadian adults get far too little physical activity. Marathon running sits at the far end of the intensity spectrum — and for most runners, the health benefits outweigh the risks. But those risks are real and largely preventable with the right preparation and medical support.

If you are training for a long-distance event and have concerns about your cardiovascular health, musculoskeletal readiness, or recovery from a prior injury, a sports medicine physician or family doctor can assess your specific situation and help you cross the finish line safely.