In practice, most of the time APTX 4869 works as a lethal poison.
It is notable that in the instances where Shinichi or Shiho were successfully changed back, the recipient was suffering from a common cold while the component was ingested, indicating that the immune system somehow has to be taxed for the antidote to work.
The only apparent canon exception was Shinichi's reversion in the London arc.
Sometime during APTX's development, the Black Organization decided to use the incomplete form as a poison for stealthy murder. 4869 can also be pronounced "Shi-Ya-Ro-Ku" (シャロク Sharoku) Throughout the series there are several instances in which the APTX was temporarily neutralized and Shinichi and Shiho were returned to their actual age forms.
The main ingredient for this reversal is a component found in baijiu, which Haibara has already incorporated in a prototype antidote for the APTX.
The Miyano parents' version of the APTX 4869 was used to kill at least one person, Kohji Haneda who was murdered 17 years ago and included on Shiho's list of APTX victims.