we have decades of evidence about weed, like all drugs its harmful if overused by a small minority of people, those with a predisposition to schizophrenia can have it bought on by heavy use of powerful skunk type weed at teenage and young adult ages, some mental issues can be excacerbated by it if they are underlying.
https://www.theguardian.com/society/2015/feb/16/skunk-cannabis-triples-risk-psychotic-episodes-study
theres an argument that those people would develope psychosis anyway, but not enough research has been done
but on the whole its far less harmful than alcahol.
most folks who smoke a lot just get the munchies and it aint exactly motivational.
it is useful for illnesses such as parkinsons and MS, as well as other conditions.
i say this as someone who smoked for 20 years, on the whole its pretty harmless.
as for opiods, they have been misprescribed for years by doctors because drug companies push them , and the synthetic shit like fentanyl is deadly stuff, the manufacturers should be held more accountable, as should the insurance industry.
they paid fines after long legal battles, but measured against the profits they made from it its a drop in the ocean.
the biggest fentanyl producer also makes the anti overdose drug, so they clean up on both ends.