Well, that's just one Google result (try "drug use portugal" or "drug use decriminalization") - I've seen several articles about drug use decreasing in the Netherlands, Czech Republic, and Portugal after each nation decriminalized some or all drugs.

I would imagine that this is because (in Portugal, at least) instead of arrest, users are required to attend counseling and support groups (and it's much easier to seek help when one needn't fear arrest). These groups are designed mostly to help people stop using drugs, and I would bet they work at least a small percent of the time.