American sprint sensation Sha'Carri Richardson confirmed Friday she tested positive for marijuana at the US Olympic athletics trials and will be barred from the 100m at the Tokyo Games.

Speaking on US Olympic telecaster NBC's "Today" show, 21-year-old Richardson said she took marijuana to cope with "a state of emotional pain" after learning of the death of her biological mother from a reporter at the US Olympic trials last month in Eugene, Oregon.

The US Anti-Doping Agency announced moments after her confession that Richardson has accepted a one-month ban from June 28, meaning her 100m victory at the trials has been cancelled.

"I would like to say to my fans, my family and my sponsors I apologize," Richardson said. "I apologize for the fact that I didn't know how to control my emotions or deal with my emotions during that time.

"I greatly apologize if I let you guys down, and I did. And I just want y'all to know this will be the last time the Olympics don't see Sha'Carri Richardson and this will be the last time the US doesn't come home with a gold medal in the 100."

Richardson had become the sixth-fastest woman ever at the distance with a time of 10.72sec at a Florida meet in April and was seen as a contender to become the first American woman to win Olympic 100m gold since Gail Devers in 1996.

While marijuana is legal in the state of Oregon, it is on the list of World Anti-Doping Agency banned substances.

"The rules are clear, but this is heartbreaking on many levels," US Anti-Doping Agency chief executive officer Travis Tygart said.

"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 might be able to participate in the 4x100m relay, but that would require her to be selected for the squad by USA Track and Field and approved by the US Olympic Committee.