Well he is not a minor but should the police ever serve a search warrant on your home, you coul be charged with possession of marijuana.
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One potential liability is that the parents could get charged with possession if it is kept in a common area. This is not likely, but it could happen. The charge would depend upon the amount. Less than 20 grams is a misdemeanor, while more is a felony. If he grows plants on the premises, that could potentially lead to felony charges being brought against the parents.
If you were not aware of him using in your home, then you shouldn't have any liability. He is an adult and he is responsible for his own actions. Of course, if you supplied, or contributed, then you could be held liable. If you were aware but did nothing, then it is possible you could get charged for not reporting a crime you were aware of. It is best for you to get an attorney to protect you not only your own rights, but your son's.
A "child" over 18 has reached his age of majority and by law is an "adult". He is responsible for his own actions. BUT, since he is living at home with his parents, if the home becomes a crime scene or subject to a search warrant and drugs are searched and seized, the homeowners could be charged as well, depending on the facts of each case. The quantity, the location(s) throughout the house, were they in plainview to all who live there, are there roaches in several ashtrays, etc.
Over zealous or cautious police officers sometimes charge everyone under a "constructive possession" theory and let the prosecutor sort the true ownership out later.
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