Short answer. Fifteen days after the last publication of the Board's notice. Section 93 provides that when a new drug is added to the list, no criminal liability arises until that fifteen-day period lapses. Until then, possessing the newly listed substance is not yet an offence under the Act.
What the law says
In case of the addition of a new drug to the list of dangerous drugs and precursors and essential chemicals, no criminal liability involving the same under this Act shall arise until after the lapse of fifteen (15) days from the last publication of such notice
RA 9165, Section 93 — Reclassifying Or Delisting A Drug. Read the full provision →
Adding a drug is a formal, published process
The list of dangerous drugs is not fixed for all time. Section 93 gives the Dangerous Drugs Board the power to reclassify, add to, or remove substances from it, after notice and hearing and after weighing factors such as a substance's potential for abuse, its pharmacological effect, and the country's treaty commitments. Crucially, the Board must give public notice of the hearing by publishing it in a newspaper of general circulation once a week for two weeks. So a substance does not silently become illegal; its addition runs through a process the public is told about in advance.
The fifteen-day rule is the answer
For the question of when possession becomes a crime, the operative words are precise. On the addition of a new drug, no criminal liability involving the same under this Act shall arise until after the lapse of fifteen (15) days from the last publication of such notice. So there is a defined grace period: the fifteen days run from the last publication, and only once they have lapsed can possession or use of the newly listed substance be prosecuted under the Act. Before that point, the substance is not yet one the drug law punishes you for holding.
Why the delay is built in
The grace period reflects a basic fairness principle: people should not be punished for handling something that was lawful when they acquired it, before they could reasonably know it had been listed. Tying criminal liability to a fixed interval after publication gives that notice a concrete cut-off rather than leaving it to argument. It also means the date of the last publication is a fact worth pinning down in any case involving a recently listed substance, because it fixes the exact day the conduct could first attract liability under the Act.
Removal runs the other way
The section is symmetrical, and the removal rule is striking. If a drug is taken off the list, all persons convicted and/or detained for the use and/or possession of such a drug shall be automatically released and all pending criminal prosecution involving such a drug under this Act shall forthwith be dismissed. Delisting is therefore not merely forward-looking; it reaches back to undo convictions and pending cases built on that substance. Anyone affected by a listing change, in either direction, should check the exact publication and promulgation dates, because those dates control the consequence.