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.