The drug the used by the Black Organization on Shinichi Kudo. Professor Agasa and Ai Haibara are currently working on a permanent cure.
This drug was created by Ai Haibara when she was still in the Organization. It is intended to kill the person who took it without leaving a trace. It luckily had the rare side-affect of turning the victim into a child instead of killing them. Professor Agasa and Ai have been able to create a temporary antidote for the poison that only works for 24 at most. Shinichi has taken this antidote on rare occasions since resistence to this drug can lessen its effects on him. The only way to have a permanent cure would be for him to get a sample of APTX-4869.