The TV presenter has lost one and a half stone since March, but she has insisted that she's healthy and denied suggestions she was "flaunting" her weight loss on Twitter.
"To those telling me I look skinny and to eat something, I can assure you I've just cut out eating crap. I still eat like a horse," she recently tweeted.
However, those close to her are worried that she is ill or over-dieting after she appeared gaunt at Fashion's Night Out on Wednesday, The Sun reports. They also noticed dark marks near the veins on her left arm
An onlooker said: "Peaches is shockingly thin and you could see what looked like bruises and red marks near the veins at the joint of her left arm.
"The marks could be entirely innocent but knowing her problems with drugs in the past, it must be a worry for everyone close to her."
Geldof overdosed on drugs in 2008, but later brushed it off as a "bad experience". Last year, she denied claims that she'd had a heroin-fuelled sex romp in the US.
Meanwhile, on Tuesday, police and ambulance crews were called to her home at around 10.30pm. They found Geldof with a man in his 30s who was "in respiratory arrest". The patient was later released from hospital after treatment and a police spokesman said that "no further action is being taken".
> Gwyneth Paltrow, X Factor' Tulisa Contostavlos, Doctor Who Karen Gillan celebrate London's Fashion Night Out - pictures