Claire Squires collapsed near Buckingham Palace last April near the end of the marathon, which she was running to raise money for an organization that helps prevent suicides.
According to an inquest at Southwark Coroner's Court, the 30-year-old runner's water bottle included a scoop of the supplement Jack3D, which contains a stimulant called DMAA or dimethylamylamine.
DMAA is on the list of substances prohibited by the World Anti-Doping Agency, but Jack3D was legally available to buy in Britain before being banned in August due to potential risks to public safety.
"DMAA ... on the balance of probabilities, in combination with extreme physical exertion, caused acute cardiac failure, which resulted in her death," coroner Philip Barlow said.
The substance was bought legally online by Squires, but the coroner said he hopes the case would highlight the potential dangers of DMAA, which increases the heart rate.
Marathon organizers are now assessing changes to the advice given to recreational runners about the use of supplements.
"The substance is on the (WADA) banned list, but the only athletes to be tested would be elite athletes," London Marathon spokeswoman Nicola Okey told The Associated Press. "We just ask the rest of the runners to be medically fit. We don't make any other inquiries about what substances they are taking.
"We will be amending our medical advice following the inquest's verdict. We obviously give medical advice, but it hasn't mentioned before the use of supplements."
Squires' boyfriend, Simon van Herrewege, said he didn't know of the dangers of consuming Jack3D and wants awareness raised.
"She innocently took a supplement which at the time was entirely legal and widely available on the high street and somewhat worryingly apparently used by so many others," he said. "It is clear that there needs to be far better supervision of the so-called health foods and supplements industry so that no more tragedies like this happen again, causing other families to have to go through what we have been through this past year."