I think this esteemed from Canelo being tested positive banned substance clenbuterol on 2 occasions. But he blamed it to the tainted meat he consumed in Mexico. This is prior to their first fight in May 2018.
Although a lot of Mexican boxers have been tested positive as well with Clen due to meat consumption as well.
Well.. you need to admit that in the world of sports, there are two set of rules. One rule for the Americans, and another one for the non-Americans. If Golovkin had tested positive for Clenbuterol, then he will be banned for life, with no questions asked and no possibility for appeal. In case of Canelo, since he is an American he managed to get away. And this happens very regularly. Remember Maria Sharapova getting banned for Meldonium, while Simone Biles was let off despite her positive test for Methylphenidate claiming that she used that drug as ADHD medication?
Of course no one would ban Canelo, even if he inject himself during weigh-ins, because he is a milking cow of boxing. Of course Canelo is taking some "extra medicine". Look at his body during his first fight against GGG, and against Plant. Skinny vs tiny bull, but the guy is in same weight class and those few lbs difference cant change body so much. Anyway, even if Canelo would be caught with something in him, GGG would still give him a good fight and competition.