Dr. David Black, CEO of Aegis Sciences Corp., which runs the testing program, said he specifically identified the drug in several conversations with the Sprint Cup driver over a three-day period after NASCAR suspended him.
"I spoke with him about his positive test result on the day he was suspended, and I spoke to him directly about the test result," Black told The Associated Press. "Yes, by name of what he tested positive for."
On Saturday, Mayfield said he was never told what drug caused the positive test and has yet to receive a copy of the results.
Black said he was not sure if Mayfield has seen the test results because a copy would be issued by NASCAR, not Aegis.
NASCAR spokesman Ramsey Poston said he was not aware of any request by Mayfield to obtain the results.
"We'd be happy to provide one if he wants it," Poston said. "We've done this in the past upon request for those who tested positive."
Mayfield maintains the use of a prescription drug with over-the-counter allergy medication Claritin-D led to his positive result.
Black has repeatedly rejected that explanation.
"I did address specifically and explicitly what he tested positive for, and that led to the discussion of his over-the-counter and prescription use," Black said. "I explained carefully and completely that his positive test result was not related to an over-the-counter or a prescription.
"They were not the cause, and could not be the cause, of his result."
NASCAR has refused to reveal the substance. Citing that policy, Black also declined to identify the drug.
NASCAR chairman Brian France has described Mayfield's test as a "serious violation" of the substance-abuse policy, and he categorized that as use of a performance-enhancer or a recreational drug. A person familiar with the test results has told the AP the positive test was not for performance-enhancers, meaning the positive test resulted from an illegal recreational drug.
Mayfield, who watched a portion of Saturday night's All-Star events from the top of a hospitality suite in the infield of Lowe's Motor Speedway, told reporters he did not use an illegal drug, insisting he used Claritin-D and a prescription drug he declined to identify.
"They didn't say what I took. They don't know what I took," Mayfield said. "A legal prescription drug, that's what I take. And I had allergies at Richmond that were really, really bad. On (May 7) I got a call and said you've tested positive for whatever they called it.
"I said, 'OK, no problem. I've got all my paperwork ready.' He said 'You'll fax that to us; it'll cancel out your test."'
Mayfield said his next conversation with NASCAR was two days later, when he was told his backup "B" sample had also tested positive and he had been suspended.
NASCAR said there were numerous conversations between Mayfield and a medical review officer before he was suspended.
Black would not say what prescription drug Mayfield said he had used in conjunction with Claritin-D.
"He did bring up Claritin-D as a part of his explanation, and I was very clear that Claritin-D was not responsible for his positive test," he said. "It could result for a positive pseudoephedrine test, but not for the drug that he's tested positive for."
Black said his last conversation with Mayfield was one week ago, and he was left with the impression the driver would participate in the reinstatement process and "move on in a positive manner."
Since then, Aegis representatives have been unsuccessful in speaking to Mayfield about starting a rehabilitation process, Black said.
Mayfield said Saturday he would not go to rehab and may seek legal action to rescind his indefinite suspension.
