Below you will find a video that discusses this issue with a "well-known" doctor. What bother me though is that two of his crew also tested positive. Do not believe in coincidences.
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http://sports.yahoo.com/nascar/blog/from_the_marbles/post/ESPN-reports-Mayfield-tested-positive-for-meth;_ylt=ApIPqDsoAI3t7zT.A89gMMnov7YF?urn=nascar,169096