The drug was developed from the spore-weed plant. For centuries, the people of Mendeb Three had used the seeds of the plant as a mild narcotic. Vethran had scientists experiment with the seeds, distilling them and combining them with other chemicals. Each result was labelled SS (for Spore Seed) and a number.
They discovered that SS10 removed a subject's free will, but did not affect their intelligence or memory. It did this partly by reducing inhibitions and increasing suggestibility, but this was a common effect of most of the distillates. What made SS10 unique was an ingredient which modified the hypothalamus. It created a cell cluster which continually manufactured another chemical which cauterised nerve endings whenever the subject desired or wanted something. This made the subject completely subservient to anyone that gave them commands. The cauterisation was irreversible, though the Seventh Doctor thought it possible that the victim's might recover partially or fully, given enough time. (PROSE: Independence Day)