Felicite Tomlinson, 18, was found unresponsive by a friend at her west London flat at lunchtime on March 13.
The pair had taken cocaine the night before and although emergency services attempted to resuscitate Miss Tomlinson, she was pronounced dead at the scene.
A toxicology report revealed there were traces of cocaine and the medications Xanax and OxyContin in her system.
Coroner Shirley Radcliffe said the combination of the drugs "proved fatal".
"It was the perfect storm," she said.
Miss Tomlinson, known to friends as Fizzy, was an aspiring fashion designer and had a history of "recreational drug use over the years [but] on a consistent basis since her mother's death" in 2016, Inner West London Coroner's Court heard.
She had purchased cocaine with with her friend, Zainab Mohammed, the evening before her death.
The inquest heard that Miss Tomlinson had taken cocaine, MDMA, acid and cannabis in the past and knew a number of people who could access these drugs.
Recounting Miss Mohammed's version of events, Dr Radcliffe said: "A male friend came over and you did lines [of cocaine], one or two, but you felt unwell.
"You saw [Felicite] do more lines but you didn't know how many she did."
Miss Tomlinson was still breathing when checked at 5am but when she woke around noon, Miss Mohammed, who attended Wednesday's hearing, "couldn't hear breathing" from the bed next to her.
Miss Mohammed only spoke to confirm the coroner's statements.
Felicite's pop-star brother had attended a number of appointments with the 18-year-old in the months before her death, the hearing was told.
Dr Paul Eulinger, her GP, said in a statement that Felicite "refused to give up drugs in the knowledge they could kill her".
After some persuasion, Miss Tomlinson completed a successful stint in an Egyptian rehab facility in Autumn 2018 and tested clean on her return to the UK on November 26, but she then relapsed in 2019.
Concluding the teenager's death was a result of misadventure and drugs toxicity, Dr Radcliffe said: "[Felicite] was a bright, much-loved daughter and sister.
"Her family clearly wanted her to engage with drug services.
"Her use of drugs was a considerable danger to her.
"She was an individual who took drugs deliberately and has succumbed to their said effects accidentally.
"There is no indication this was a deliberate attempt to end her life."
Paying tribute at the time of her death, Felicite's sister, Phoebe said: " You were my best friend, sister, motivator and person that could make me laugh until I cried."