World 200 metres champion John Capel did not run on the United States 4x100 metres relay team in Friday's Olympic semi-finals because of a positive test for marijuana, U.S. head men's coach George Williams said on Saturday.

"Why take a chance?" Williams told Reuters in a telephone interview. "I did not want to risk anything for him or the other three on the relay."

Capel tested positive at a meeting in Munich on August 9.

Williams said he found out about the positive test only an hour-and-a-half before the U.S. team ran on Friday.

A positive test for marijuana carries only a public warning and Capel would have been eligible to run in the relay but Williams decided not to use him.

"Once it (the positive test) was out, I decided to let him rest," Williams said.

Olympic 200 metres champion Shawn Crawford replaced Capel and the team, with Darvis Patton, Coby Miller and Maurice Greene, won their semi-final in 38.02 seconds.

Olympic 100 metres champion Justin Gatlin was scheduled to replace Patton in Saturday's final.