I'm sure smoking cigarettes should be okay. As for smoking weed it can intensify whatever you are feeling at the moment, so you will probaly get really panicky when the survival instinct kicks in. But others on here have said it won't interfere with the SN process besides that. Even Stan from Stan's guide said that weed has mild AE properties:
https://sanctioned-suicide.net/threads/would-weed-work-with-the-sn-method.26664/
Here's what pops up in the search results if you want to read more:
https://sanctioned-suicide.net/search/681409/?q=SN+weed&o=relevance
But in my opinion atleast for me even if I had some I wouldn't do it cause of how paranoid I'd get.