Sha'Carri Richardson, the US champion and second-fastest 100 metres sprinter in the world this year, will not contest the event at the Tokyo Olympics after receiving a one-month ban for cannabis use.
The positive test means her victory at the US Olympic trials last month has been wiped out, leaving her ineligible for the 100m at the Tokyo Games. There remains a chance she could still be selected for the 4x100m relay.
Richardson, 21, has burst onto the international stage this year, quickly establishing herself as a potential gold medal contender after running a personal best 10.72 seconds in April.
Speaking on Friday, Richardson - who was raised by her grandmother - said she used cannabis after finding out her biological mother had died a few days before the US trials.
Cannabis is legal in Oregon, where the competition took place, but the substance is banned in professional athletics under anti-doping rules.
"I want to take responsibility for my actions," she told NBC. "I know what I did, I know what I'm supposed to do and I still made that decision.
"I'm not making any excuse or looking for any empathy in my case. However, being in that position of my life, finding out something like that, was something that I would say is one of the biggest things to impact me, when it comes to dealing with what happened with my mother.
"That definitely was a very heavy topic on me and people don't understand what it's like to have to put on a face in front of the world, hide my pain.
"Who am I to tell you how to cope when you're dealing with the pain or a struggle that you've never experienced before?"
She added: "I apologise for the fact that I didn't know how to control my emotions during that time. I'm human.
"I want to be as transparent as possible. When it comes to Sha'Carri Richardson it will never be a steroid. There will never be a steroid attached to the name Sha'Carri Richardson. It was marijuana.
"I'm not encouraging anyone to do it. Don't judge me because I am human. I am you, I just happen to run a little faster."
Confirming the one-month ban, US Anti-Doping Agency chief executive Travis Tygart said: "The rules are clear, but this is heartbreaking on many levels; hopefully, her acceptance of responsibility and apology will be an important example to us all that we can successfully overcome our regrettable decisions, despite the costly consequences of this one to her."
Richardson had previously faced questions about her associations with convicted dopers after joining coach Dennis Mitchell's group, where Justin Gatlin is her training partner. Both men have served drugs bans.
Her potential relay involvement in Tokyo depends on selection for the US team, but her guaranteed 100m absence aids the likes of Britain's Dina Asher-Smith, who is hoping to replicate her performance from the last World Championships and make the Olympic podium.
The news has prompted a wave of reaction about marijuana remaining a banned substance.