WA doctors are urging competitors in fun runs to go easy on the "no guts, no glory" motto and stop if they have symptoms such as sudden chest pain.
The Australian Medical Association WA has developed new safety guidelines for participants, including a pre-event medical checklist and warning signs for when people should stop exercising.
It comes after the deaths of two participants in this year's City to Surf in August.
Dentist Jeffrey On, 32, died in hospital from a suspected heart attack soon after finishing the 12km run and 58-year-old lawyer Daniel O'Dea collapsed soon after finishing his first attempt at the half-marathon and died at home two days later.
AMA WA president Richard Choong said the new advice for participants would ensure they were properly prepared for the exertion required.
People are advised to slow down during the event if their heart starts racing or they feel dizzy or sick, and to stop immediately if they have sudden and severe chest pain.
"Fun runs are great initiatives, with hundreds of thousands of people taking part each year across our nation," Dr Choong said.
"But we cannot afford to ignore the potentially fatal health impact of such exertion."
The AMA wants the guidelines to be given out to all people who register for fun runs.
"Where an event requires high physical exertion, and attracts novice exercisers, there must be comprehensive health information provided to anyone registering in the event," Dr Choong said.
"Participants need to be aware of the risks involved when taking part in activities that require high levels of endurance and ensure they are properly prepared for the event.
"This is especially important for individuals with a history of chronic medical issues, such as diabetes, high blood pressure or arthritis, or high-risk factors such as smoking, being overweight or having a family history of heart disease or sudden death at an early age."
He said anyone with doubts about their fitness or a family history of illness, and those returning to running after a major break, should check with their GP before the event.
The guidelines are available at amawa.com.au.