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Marijuana can be very toxic to dogs, you will need to get him in to see his vet asap. Marijuana can cause Prolonged CNS depression, respiratory depression, weakness, ataxia, sedation, and sometimes hyperexcitation.
Do not worry about the vet calling the police, they are not interested in recreational drug use. If it makes you feel better you can tell him that he picked it up at the local park. I would suggest getting him in tonight, you will need to watch him for signs of toxic reaction such as vomiting, diarrhea, excessive drolling, trouble breathing or dizzy confused actions.