Poorly worded. I think what he meant to say was, "Marijuana is a psychoactive drug that has an effect on the brain. Therefore, I prefer to protect my most valuable asset against inadvertent harm." :)

Do you ever stay up too late? You're harming your brain.

Just to be clear, I was translating, not giving my own views on the subject.

Since harm is cumulative, the more harmful behaviours I already partake in, the less desirable it is for me to add more, no?