The test failure was for hCG, a substance often used as someone cycles off synthetic testosterone to kick natural testosterone function back into gear. Siver discussed the situation with Groundandpound.de and still seems a bit confused about the situation:
Dennis Siver on his positive drug test after UFC 168:
"Before my last fight, I took a medium to help me with my diet, hCG drops. This is an over the counter product, you can buy it on the internet. It’s nothing bad, it was supposed to help burning fat and it helps suppressing the hungry feeling. That’s why I tried it, and unfortunately, it came up in the drug test. There was a small value in my sample, but it was enough to suspend me indefinitely until the situation is going to be dissolved.
And unfortunately, everything is taking too long, in my opinion, because normally, it should be dealt with within two or three weeks and then you know where you’re at. In my case, this thing has taken three months already and it continues to get pushed back more and more by the athletic commission. Nobody is able to do anything about it, neither me nor the UFC. That’s why I won’t be able to fight in Berlin. Even if the punishment will be minor or there will be no punishment at all, there’s not enough time to prepare for a fight. That’s why I’ll have to fight somewhere else."
He went on to say that his trainer was ill and he used a new guy who suggested the hCG diet to lose weight and, because the drops didn't say 'banned for athletes,' he assumed they were fine to take.
This serves as a good reminder for fighters to check with commissions where you will be fighting, as their list of banned substances is usually quite clear.