Anderson Silva is often seen as a legend and one of the best fighters in UFC history. Unfortunately, his reputation was hurt after failing drug tests on two separate occasions.
To make matters worse, Dillashaw popped for recombinant human erythropoietin (rHuEPO) aka EPO. This is widely regarded as one of the worst drugs an MMA fighter can test positive for in the spirit of fair competition. Here is USADA's brief explanation of the drug, and what it does to benefit athletes.