Counter is that it reduces the population of users. Simply diverting funds entirely to helping users would seem to invite perpetual spend.
I'm not even sure how one would go about measuring it, given all the other initiatives happening concurrently with this ad campaign.
Intuitively, something a bit attention-grabbing like this could reach more people than some stodgy "drugs bad mmmkay" thing, and I don't think it's wrong to spend a small percentage of a large harm-reduction budget on an awareness campaign. But that's about all I'd be confident saying.