The 30-year-old from North Kilworth, Leicestershire collapsed and died while running the final stretch of the 26.2-mile London Marathon course in April last year.
Her boyfriend Simon van Herrewege told the Southwark Coroner's Court that she had bought a supplement called Jack3D, which contained heart-rate increasing stimulant DMAA.
"Claire ended up getting a tub of this supplement Jack3D to give you a bit of an energy boost," van Herrewege said.
"It is a bit like caffeine. Claire never really got on with it. She never really liked it.
"She said she would take one scoop in her water bottle.
"She said that if 'I hit a bit of wall I will take it'."
The hearing was told the supplement can still be bought on Amazon but that DMAA has since been removed as an ingredient.
Dr Jon van der Walt, who performed the post mortem, suggested that the cause of death could have been "extreme physical exertion complicated by DMAA".
She had been running the marathon with the aim of raising just £500 but as news of her death spread, tens of thousands of pounds was donated to her Just Giving page - and it kept on coming.
More than 80,000 people have made a donation on her page – raising more than £940,000 – with donations from other sources taking that total to well over a million.
Squires, who had been diagnosed with an irregular heartbeat, was running the London Marathon for the second time when she fell to the ground on Birdcage Walk, near Buckingham Palace.
She had been treated by medical teams situated along the route before she was rushed to hospital, where she was later pronounced dead.
Squires was running to raise funds for the Samaritans in honour of her brother Grant, who died of a drug overdose in 2001 aged 25.
Her mother Cilla has also volunteered for the Samaritans for over 24 years and has since worked with the charity to develop The Claire Squires Fund to help out charities they believe Claire would have been passionate about.
Her family and friends have also set up the Squires Effect charity which has a similar motive.