I think you're touching on a good point about advertising in general. As nostr grows and becomes a larger audience it is going to attract all sorts of crap.

At the end of the day it's not up to you how people interact with your post. It will come down to what other users choose to allow show up in their own feed. There will be some clients that will filter out advertisements and other content and some that don't. People who don't want to see ads for drugs will choose the ones with filters that block them. People that do want drug ads will choose other clients, and you shouldn't be worried about what they see because that is their choice.