NASCAR has indefinitely suspended AJ Allmendinger after the second-tier results of a drug test were returned yesterday with a positive result.

Before his suspension, Allmendinger, 30, drove for Penske Racing.

He was subjected to a random drug test on June 29 in Kentucky, where he was about to drive in the Sprint Cup Race at Daytona International Speedway. He was immediately pulled from the race.

Urine specimens are split into two samples for testing. When he tested positive for an undisclosed banned substance, Pennske owners said they would reserve judgment until results from the "B" sample were returned.

Allmendinger has said that he did not knowingly take an illicit substance, and speculated that it had probably entered his system in a nutritional supplement that he took for his fitness regimen.

"AJ is collecting his medicines and supplements for testing to determine whether an over-the-counter product caused his positive test," his business manager, Tara Ragan, said.

With the B sample result coming in positive yesterday, Allmendinger's racing career may be over.

On Twitter yesterday the driver said, ""I just want to say thank you first and foremost for all if u sticking by me....I'm sorry we even have to have this going on. But i promise....i will do whatever it takes to get to the bottom of this and get back our [sic] there no matter what."

In 2009, NASCAR driver Jeremy Mayfield was granted the right in a federal court to compete again, having been banned for failing a drug test.

Mayfield's reinstatement was brief; he afterwards again failed drug tests, and his ban remains in force.

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