Obvious solution is to make soft drugs like fentanyl fully legal. Just apply sufficient purity to standards as regulation like with any other drug. Issue solved instantly. It is not like other deathly drugs aren't already legal. [emphasis mine]

sarcasm or typo?

Fentanyl is not a soft drug, but opium, heroin and morphine can be so if administered correctly, and meet the same needs satisfactorily. Fentanyl is extremely deadly if misdosed.