Williams had been using a substance which was "clearly outside the medical code," St. Kitts Olympic committee vice president Dennis Knight told The Associated Press on Sunday.
Knight said Williams had not tested positive, but the team acted after consulting with the World Anti-Doping Agency "to find out about the product."
"In discussions with our team management, she volunteered to them that she had been using a particular substance which, when we did our own investigations, we considered to be outside the accepted medical code," Knight said.
Williams told team officials about using the substance -- which the team has not disclosed -- in a pre-Olympics training camp.
"It was a matter of the management of the team doing their due diligence," Knight said.
The 22-year-old Williams had qualified for the 100 and 200 meters, and gave samples for anti-doping tests at national Olympic trials last month.
"It was not based on any positive drug test. She turned up a clean test," Knight said.
Williams marched at the Olympic opening ceremony on Friday as the only woman in a seven-member team from the Caribbean islands, who are all track sprinters. The best known is five-time Olympian Kim Collins.
Knight said St. Kitts team officials sought expert advice in London before acting.
"We wanted to consult with the anti-doping fraternity," he said. "We are a very tiny country with limited knowledge of these things."