Williams met April 10 with NFL counsel Jeff Pash in an attempt to have the league overturn the test. He had been participating in the offseason training program at the Dolphins' complex, and he was there working out shortly before league announced that his appeal had been rejected.
"I'm disappointed with the decision, but I respect it," the 2002 NFL rushing champion said in a statement released by the team. "I'm proud of my association with the National Football League and look forward to returning to the Dolphins in 2007."
Williams also sat out the 2004 season after retiring shortly before training camp. He returned last year to play for new Miami coach Nick Saban.
Williams' previous positive drug tests were for marijuana, which he acknowledged using. The latest test apparently involved a substance other than marijuana and may have been related to his interest in holistic medicine.
Since his return last season, the NFL required Williams to undergo drug tests up to 10 times a month. He was in India studying yoga when news of his latest failed test surfaced in February.
Williams served a four-game suspension at the start of the 2005 season for his third violation of the drug program, then ran for 743 yards and averaged 4.4 yards a carry while sharing playing time with rookie Ronnie Brown.
Saban repeatedly has praised Williams' conduct and performance last season and supported him in the appeal process.
"This is a league decision, and we are disappointed in what it means for Ricky and the team," Saban said in a statement. "Ricky did an outstanding job for the Dolphins, not only as a player but also what he added as a person to the team's chemistry and to our overall success."
One sliver of good news for Saban is the timing of the decision: He now knows he's in the market for a running back in the NFL draft this weekend to back up Brown.
The suspension represents a financial blow for Williams, who owes the Dolphins $8.6 million for breaching his contract when he retired in 2004. His return last season was motivated partly by the need for a paycheck, and that may be a reason for him to return in 2007.
Attorney David Cornwell represented Williams in his appeal.
"We raised substantial and legitimate issues arising out of the application of the NFL's policy and program for substances of abuse," Cornwell said in a statement. He urged the players' union and ownership to "review the issues we raised on the appeal ... and restore the original intention of the NFL's policy to put equal focus on helping NFL players as is put on testing and suspending them."
Last season Williams laughingly described himself as weird and compared his career to a roller coaster. But he also dispelled his reputation as an aloof, selfish pothead, winning praise from teammates and winning the local media's annual postseason Good Guy Award, given to the player most cooperative with reporters.
Williams won the 1998 Heisman Trophy at Texas, and the next year, New Orleans Saints coach Mike Ditka traded his team's entire draft to acquire the running back. Williams wore a wedding dress for an infamous photo shoot with Ditka upon arriving in New Orleans, and the resulting backlash began to sour Williams' three-year stay with the Saints.
The Dolphins made their biggest trade in more than 30 years to acquire him in 2002, giving up two first-round draft picks, and he rushed for 1,853 yards that season.
Now his career could be over. If Williams does try to return next year, he'll be 30 years old and will have played a total of 12 games in the previous three seasons.
"Ricky is obviously disappointed," said his agent, Leigh Steinberg. "He'll need to work hard to get back to the league in 2007."