I have been very suspicious of Costa up until now. Take a look at Costa's drug test history -- tested six times in 2018 and 17 times total since the USADA program began.
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Here we see he's been tested 3 times under a different name. Conspiracy theorists can run with that but I doubt there's anything shady about it.
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Unless he's taking some peptide that's undetectable thus far, than maybe Costa is legit natty. Or maybe USADA needs to freeze blood samples and retest them as detection technology improves. I doubt USADA would want to destroy the legacy of UFC's elite fighters, though.