Powell could have been picked to compete in the relay in Moscow but acknowledged that now wasn't a possibility.
Simpson, in a statement also reported by the Gleaner, said she tested positive for oxilofrine.
She won gold at the 2004 Olympics in Athens in the 4x100-meter relay.
"As an athlete, I know I am responsible for whatever goes into my body," the 28-year-old said. "I would not intentionally take an illegal substance of any form into my system."