I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
The police are unfortunately correct. By allowing someone to move in with you indefinitely, you created an oral lease, and that gives you friend the protection of landlord/tenancy laws. This would be true in most cases even if you hadn't accepted his food card to help pay for the food he ate while living with you, but taking those contributions toward the household also establish a landlord/tenant relationship.
You do have the ability to give him a three day notice to cease using drugs in your home or vacate the property. Then, if he doesn't do it, you can go to the local landlord/tenant court and file for eviction. The problem is, if he stops using drugs, and then starts again, you have to give another notice. That can create a really frustrating cycle for you as someone who lives in the house. If you simply want him out, one way or another, the way to do that is by serving him with a 30 day notice. A 30 day notice is only valid at the end of the month, so you'd have to give the notice before Sept 30 to ensure that he is out by the end of October. A tenant in Maryland cannot be compelled to leave mid-month with a 30 day notice. So keep that in mind. And, again, if he doesn't leave, you'll need to get a court order to have him forcibly removed. That's done by filing an eviction action.
I apologize that this was probably not the Answer you were hoping to receive. However, it would be unfair to you and unprofessional of me were I to provide you with anything less than truthful and honest information. I hope you understand.