I have read that some overdoses occur when an addict gets clean for a while then relapses. They think they can tolerate the same dosage they were taking when they quit but their body can't handle tolerate it now, ie, they have to work back up to that dosage. Not sure how common that is, but better labeling and higher-quality drugs wouldn't help, though I agree, letting pharma companies manufacture street drugs and selling them legally would be a good thing.