I agree with the commenter who suggested dropping the baggie by the patient where they could see it but you wouldn't be giving it back to them. Calling the police for anything less than an amount used for dealing shows you are allied with them and may mean that people who have OD'ed or are in real trouble will not call the ambulance service. Dropping it down the drain is taking the moral high ground and is only likely to aggravate people who might already be feeling somewhat aggressive and may aggravate the situation. Whatever you do though you shouldn't carry it on you - it can only lead to trouble.