I live in flats on the first floor, with people below and above me.
About once a week I have to endure the smell of weed coming in to my bathroom/toilet.
Should I report it to the landlord or police or do nothing about it?
No! you'll just get persecuted, at worst, and at best you've shopped someone for doing what everyone almost does these days. There's probably more to this than you're saying, or you're so young and naive you think the law is there to be applied and serves justice. (No, and no.)
No. I am not young nor naive. I just find the smell of weed to be disgusting and it makes me feel sick. As for the law!!! I hate the police and am aware how justice and law are disconnected and still feel angry how one of my ex girlfriends got 12 months in jail for a technically illegal activity and the police offered the other guilty persons immunity from prosecution if they would testify against her.