APTX 4869
APTX 4869 (四八六九 Yon-Hachi-Roku-Kyuu), code named Prototype Detective, is a fictional toxin is a derivative of an experimental immortality formula developed by Shiho Miyano, which is used by the Black Organization as an untraceable, lethal poison. APTX is an abbreviation of "Apoptoxin," and, as the name implies, it involves the activation of uncontrolled apoptosis which causes death by the destruction of cells. In addition, it can also activate telomerase.
However, in very rare cases, it causes the apoptosis of non-neural cells only and the apoptosis would stabilize at some point, causing the victim's apparent age to be decreased. Shinichi, the series' protagonist, is such a victim; later Shiho Miyano, who chooses to attempt suicide by using her own drug, was added to the two known people to have survive after taking the APTX-4869 throughout the series.
Reversing the effects
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 Chinese ginseng root schnaps, which Ai has already incorporated in a prototype antidote for the APTX.
It is to note 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. While not canon, Crossroad in the Ancient Capital uses this idea as Conan is given a pill to give him the symptoms of a cold, while drinking Baigar.