What makes you think any of us were using the lay definition?

Because if someone said “opioids are neurotoxic” from a scientific definition, it’s stating the obvious.

Both definitions are correct.

From Wikipedia:

> Neurotoxicity is a form of toxicity in which a biological, chemical, or physical agent produces an adverse effect on the structure or function of the central and/or peripheral nervous system. It occurs when exposure to a substance – specifically, a neurotoxin or neurotoxicant– alters the normal activity of the nervous system in such a way as to cause permanent or reversible damage to nervous tissue.

Brain cells getting killed is a normal process, you could call it pruning. Cells grow back, you know!