she probably honestly believes what she said.... nothing you can really do...

you're 20 dude.... put your foot down... tell her it's your life, you're not shooting up or smoking crack. marijuana is well on it's way to being decriminalized and this isn't the 50's or 60's... that you understand how she feels, because she grew up in an era where it wasn't studied (or atleast publicly) and it wasn't anywhere nearly as widely accepted as it is today... that getting caught these days is usually no worse then getting a couple of traffic tickets so long as you're not holding on to ounces or pounds of the stuff...

tell her of the medicinal benifits, as well as the theraputic ones. that there is nothing in it to be adictive (or atleast at the level of alcohol or tobacco) and it doesn't even cause cancer (infact the oppposite)

tell her how you're responsible, you don't sit on your ass all day smoking the day away, you use it about ht esame as a casual drinker would drink, at a far less taxing price of your body.

then drop the bomb on her... ask her if she would really freak out so bad if she found out you had a few shots or beers on the weekend.... and if not why she's taking it this way...