I smoke weed but I'd NEVER EVER let my animals anywhere near it. I don't think it's a "drug" as such, more like tobacco or alcohol but less damaging. I think that if you can't guarantee that your bunny will be safe in that apartment then I wouldn't keep her there. I'd rehome her to a good place for her own good, the fact that she was bought for you for 19th birthday present shows that the boys you live with have no idea of what kind of responsibility a rabbit (or any animal) is.
Where do you live? The only place I know where it is smoked legally is for medical reasons or only allowed in cafes or something. I don't know everywhere though, so maybe it's legal to buy for personal use at home for recreation reasons in other countries.
Paracetemol and other things are legal here but I would still be shocked if somebody left it lying around whilst a bunny was in the house. The lads you live with know there is a bunny and tbh I'm shocked that they left it there for her to get.
I'm wondering at 19 if you are able to afford her being spayed, vaccinated and regular vet checks? (don't take this the wrong way but I know when I was 19 I couldn't afford to feed myself let alone any animals!)