when the state criminalizes the selling of a drug that nevertheless has a significant demand from citizens who want to use it, that effectively legislates a monopoly for criminal organizations to sell that drug at outrageously steep prices. It creates an extremely lucrative business for criminals to run.

Additionally, the state's attempts at enforcing the ban will prevent the free market competition process from improving the product. Thus amplifying the risks to users.

The steep prices due to government enforcement will also be very difficult to afford, leading to addicted users having no way aside from theft to fund their addiction. This creates a lot of unnecessary property loss for people who have nothing to do with the drug market. Property loss that just wouldn't happen if selling the drug wasn't criminalized.

Also, it's at best ineffective in curtailing the use of the drug. At worst, it increases the use of the drug in practise due to young people's tendency to consider doing forbidden things cool.