Jeremy Mayfield was suspended indefinitely by NASCAR on Saturday for failing a random drug test, becoming the first driver to violate a toughened new policy that went into effect this season.
Mayfield tested positive for a banned substance last weekend at Richmond International Raceway.
"In my case, I believe that the combination of a prescribed medicine and an over the counter medicine reacted together and resulted in a positive drug test," Mayfield said in a statement. "My doctor and I are working with both Dr. (David) Black and NASCAR to resolve this matter."
Black of Aegis Sciences Corp. in Nashville, Tenn., has worked on testing programs with the NFL, Indy Car and more than 70 NCAA Division I colleges.
NASCAR spokesman Jim Hunter would not reveal what banned substance Mayfield used, but Hunter said it was not an alcohol-related offense.
"There is no place for substance abuse in our sport," Hunter said.
NASCAR also suspended two crew members for failed tests at Richmond.
Tony Martin, a crew member for the car John Andretti drove last weekend at Richmond, and Ben Williams, a crew member for the Nationwide Series car Matt Kenseth drove last weekend, were both suspended indefinitely.
Mayfield, who is driving a car this season he owns himself, failed to qualify for Saturday night's Sprint Cup race at Darlington Speedway.
The suspension applies to Mayfield's roles as owner and driver of the No. 41 Toyota. Although the car can race next week at Lowe's Motor Speedway with another driver, Hunter said it cannot be entered with Mayfield as the owner.
Mayfield said in his statement that an interim owner and a temporary replacement driver would be announced early next week.
John Andretti, who finished 32nd in last week's race at Richmond, said he's not worried that the driver next to him might be driving impaired and applauded NASCAR's tougher drug policy.
"I think it's a great thing that they (NASCAR) do," Andretti said from Indianapolis, where he's preparing for the Indy 500 later this month. "And whoever they catch and confirm, so there's no mistake, shame on them."
Just days after the Daytona 500, one of Mayfield's crew members became the first person punished under NASCAR's new drug policy for a failed test. Mayfield fired Paul Chodora after he was suspended by NASCAR.
"We as an organization appreciate NASCAR's drug testing policies and policing efforts as it makes the sport stronger overall," Mayfield said after firing Chodora. "If Paul doesn't comply with NASCAR's reinstatement process, then he will no longer be an employee of Mayfield Motorsports."
Mayfield, a two-time qualifier for the Chase for the championship, has five Cup victories in 433 career starts, but none since 2005 at Michigan. He was fired by Evernham Motorsports in late 2006 and bounced around until this season, when he formed Mayfield Motorsports.
He threw the team together in less than a month but made headlines as the underdog who raced his way into the season-opening Daytona 500. But he made just four of the next 10 races, and is currently 44th in the Cup standings.
NASCAR announced a new, tougher drug policy last September. The guidelines were strengthened in part because of former Truck Series driver Aaron Fike's admission that he had used heroin _ even on days he raced. That led Tony Stewart, Kevin Harvick and other veteran drivers to call on NASCAR to add random drug testing to its policy.
Under the new rules, all drivers and crew members had to be tested before the season began. Random tests are scheduled throughout the year, and at least four drivers are tested each weekend. Hunter said the drivers are selected through an automated computer program.
Former NASCAR driver Dario Franchitti was stunned by the news.
"I know the IndyCar drug policy is pretty stringent, and I know NASCAR has really been ramping it up," he said from Indianapolis, where he qualified third. "I think it's very important when you're in a car that you have to be there 100 percent."
Trucks driver Ron Hornaday last year admitted using testosterone for more than a year _ before it was added to the sport's banned list _ to treat a medical issue. Hornaday has Grave's disease, a condition he is now treating with Synthroid, which replaces a hormone normally produced by the thyroid gland to regulate the body's energy and metabolism.
NASCAR did not punish him for the testosterone admission because the cream did not enhance his performance or impair his judgment.
NASCAR's past policy allowed for testing any time series officials had "reasonable suspicion" to question a driver or crew member. Fike's admission forced NASCAR to begin a weekly, random process.
On Wednesday, former Nationwide Series driver Kevin Grubb was found dead in a Richmond-area motel room from what police said was an apparent self-inflicted gunshot wound to the head. Grubb was suspended indefinitely by NASCAR after a second failed drug test in 2006 and never raced again in a NASCAR sanctioned event.
Shane Hmiel, who made 119 starts in NASCAR's top three national series, received a lifetime suspension in 2006 after a third failed drug test. Hmiel, who made seven Cup starts in 2004 and 2005, won the Truck Series race at Las Vegas in 2004.
Associated Press Sports Writer Mike Marot in Indianapolis contributed to this report.