the issue with a defensive shooting (in your home) with pot, (where legal) or alcohol, is not the legality of possession/use, but how it affects your judgement.
leaving aside (for now) the legal or illegal pot user/gun owner issue, getting high at home, smoking or drinking, and you shoot a bad guy who broke in...
Now, having a drink or three (or a smoke) is legal, how has being under the influence affected your judgement, when it comes to pulling the trigger? That is someting the authorities will be very interested in.
Not breaking any law being legally drunk at home, but did you break a law when your "impaired" judgement decided you had to shoot someone?
What might be a clear case of a good shoot for a sober person, could become very muddled legal ground if you are drunk/stoned. Impaired judgement could easily lead you to think it reasonable and prudent to take a step that moves you from a legally justified shooting to one that is not. And the prosecutor or the "vicitm's" lawyer will be certain to consider that, and try to make whatever you did the wrong thing in the eyes of the court/jury.
Its a very slippery slope. Suppose you (legally) smoked yesterday, and tonight you have to shoot to save your life. Pot will be detectable in your system, so it becomes a matter of interpretation if you were "impaired".
Absent a fixed legal standard of what amount of drug constitutes legal impairment, you are hanging in the wind.
And then there is also the whole argument of being actually impaired at a level above or below the legal standard. Drugs (including alcohol) affect people differently. Look at drunk driving, for instance. Some people are capable of driving perfect fine above the legal limit (although this may be a fact, it does not negate the legal "impaired" definition) and some will be impaired well below the legal limit.
example, if you are showing signs of being drunk/impaired (erratic driving, slurred speech, etc) but your blood alcohol is .04 and the legal limit is .08, you can still be convicted. Likewise, if your BA is .12 but you aren't showing any signs of impairment, you will still be convicted.
When the set a legal impairment level for pot, it will be the same thing. Maybe worse.
All else being equal (and it almost never is) bigger bullets tend to work better.