Burn out

[Kit]

Some one who smokes so much pot that they become constantly slow and dumb

[Zipporah Schumaker]

long term physical and mental exhaustion associated with a task (usually monotonous/repetitive tasks or ones careers).

a person who has smoked so much marijuana that they appear slow, sluggish, and stupid.

the temporary unpleasant, sluggish feeling after the high from marijuana wears off.

[Sommer]

to live your life to the fullest extent at a very early time in your life. (usually involves drugs.)

[Gregory]

If a fire burns out, it stops producing flames because nothing remains that can burn.