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Presumably, there's a good reason that this particular drug is prohibited. But if you think otherwise, the time for arguing against the prohibition is before you're caught, not aferwards.
COC was subject to the same rules as everyone else, so if they can ensure that their horses are drug free for the competition, it's hard to see how he can't do likewise.
And also, it's not the first time a horse of his has been found positive in competition, so claims of innocence of this occasion would tend to carry less weight.