UFC's Stephan Bonnar -- Juiced Up During Anderson Silva Fight

Sometimes steroids just don't do the trick ... just ask UFC's Stephan Bonnar, who got crushed by Anderson Silva during last month's UFC fight.

After Bonnar's loss -- which triggered his retirement from the UFC -- he was tested and Drostanolone (an anabolic steroid) showed up in his system ... according to MMAjunkie.com.

Here's the thing ... this isn't the first time this has happened to to Bonnar. He tested positive for a steroid substance back in 2006 after UFC 62. He was suspended and fined.