For the commission of a crime, there must be a guilty mind along with the physical conduct of committing a criminal act.
In simple words guilty mind + guilty act = crime
Thus, even for the crime of possession of drugs the mental intent of such possession must be present for the court to hold you guilty. This is simple words implies that there must be an intention to commit such an act and you must be aware of the fact that such activity is prohibited by law, and you carry out the act with complete knowledge and intention of its nature and the said act must be carried out by you with a free will (without the use of force). This means that a person is aware of the fact that the activities carried out by them is wrongful.
E.g. A person who accidentally hits a pedestrian with no intention of doing the same is not liable for murder. However, in the same situation if a person intentionally runs over someone with their car it amounts to the crime of murder. Thus, for the crime of drug possession, the act must be carried out intentionally with full knowledge of doing the same. It must be established that you were in the custody of such drugs and you had full knowledge and intention of doing the same. If you did not have any knowledge or intention of having the drugs in your possession you cannot be held liable for the crime.
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