Criminalizing drugs drives producers and users to more potent and compact forms, like opium gave way to heroin which gave way to fentanyl. (Very simplified.)

Opium is bulky.

When the US outlawed alcohol for a while, the market also switched from relatively bulky beer to harder spirits.

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Aside from that, opium and fentanyl etc obviously have some good aspects, otherwise people wouldn't take them. Duh.

That doesn't mean that the good aspects outweigh the bad aspects in someone else's opinion.