There's nothing morally wrong, but there's definitely something physically wrong with taking fentanyl when you don't need fentanyl, especially long term. Like just because someone is addicted doesn't mean they should stay addicted. I doubt you're advocating for alcoholics to remain alcoholic for example.

Also, I think you mean anyone against the usage of all drugs don't have a valid opinion, not the creation of all drugs (there's no good reason Krokodil should exist for example).