A young woman who died in the London Marathon was using a stimulant which has since been banned, an inquest has heard.
After Claire Squires, 30, collapsed and died in the run last April, a public outpouring of support led to more than £1m being donated to her chosen charity.
The inquest at Southwark Coroner's Court heard that Miss Squires died from acute cardiac failure.
On the day of the race, Miss Squires had put Jack3d in her water bottle, the hearing was told.
The popular stimulant used to contain an amphetamine-like substance DMAA, and experts say this contributed to her death.
DMAA was banned from shelves and internet sales in the UK in August last year over fears it could be lethal when used during rigorous activity.
Jack3d since removed the ingredient from their product.
The court heard how several US soldiers had died after taking DMAA during training, and it was subsequently banned by the US Army.
It was legal in the UK at the time Claire was using it.
Her boyfriend, Simon Van Herrewage, said she had not used it much before, but that she used it for a boost during the London Marathon.
"She put one scoop in her water bottle and said she would only use it if she hit a wall," he said.
"She was trying to beat her previous time and do it in under four hours."
In a statement, cardiologist William McKenna said: "The presence of a substance found in the blood was an important factor in the outcome."
Dr Nicola Drake, who treated Miss Squires at St Thomas' Hospital, told the court that the drug makes "heart levels increase".
"It makes the heart work harder than it normally would," she said.
Dr John Van der Walt, who conducted the post mortem, concluded that "acute cardiac failure due to extreme physical exertion complicated by the presence of DMAA was the cause of death."
Miss Squires' boyfriend also told the hearing that she had been diagnosed with an irregular heartbeat by an acupuncturist.
But her GP told the inquest that she never went to see him with concerns over her heartbeat.
Miss Squires collapsed and died a mile from the finish line of the London marathon last April.
The 30-year-old hairdresser from North Kilworth, Leicestershire, was running to raise funds for the Samaritans in honour of her brother Grant, who died of a drug overdose in 2001, aged 25.
Before the race she had raised £500 but following news of her death, thousands of well-wishers have left donations of more than £1m.
She was treated by medical teams positioned along the route before she was rushed to hospital by ambulance where she was later pronounced dead.
She had previously undertaken a string of gruelling physical challenges for charity, including a marathon in Belfast and the Great North Run.
Asked how long she trained for the London Marathon, her boyfriend said "she never stopped training".
"She ran five times a week and she knew how to build up for a marathon."
After the inquest Mr Van Herrewage said: "This has been an extremely difficult time for us, Claire's death has left a gaping hole in our hearts and in our lives.
"We'll always remember her with a big smile on her face trying to make everyone she met feel good about themselves.
"The outcome of the inquest left us feeling a little numb, Claire took part in the marathon to do some good and challenge herself in the same way she did for many other events.
"Claire was passionately against the use of drugs and would never ever have taken anything that would have caused her harm or even worse risk to life.
"She innocently took a supplement that at the time was entirely legal and widely available on the high street, and somewhat worryingly apparently used by so many others."