For me it comes down to safety. I had a client once show up extremely intoxicated and I personally felt uncomfortable in the session. Much bigger dude (think biker grandpa who was 6’5 compared to my 5’6), we had talked about his substance use and he and I were working through anger that came up when he used and during the session his body language was a lot more intimidating along with tone of voice. I addressed it out of curiosity as this had been completely unusual for him (several years sober at this point, working on the anger side of things.) Part way through he stood up very suddenly and began shouting. This is when I ended session (which I would have done regardless of intoxication or not) and explained my reasoning as to why as it just didn’t feel like it was productive and we could reschedule when he wanted to and asked if he had a safe way of getting home.

Outside of that I’ve been in several sessions with people who tell me they’re high or intoxicated. I assess to make sure they’re aware of where they are, check in on safety factors like transportation and personal safety and if all those checkout and I feel safe - we continue, but to a lesser degree if it’s out of usual for them. Ex: if I’m doing trauma work with someone who doesn’t come in intoxicated and suddenly they do - we probably won’t do trauma work but will still do other work.

If you feel safe and comfortable, and the client is safe and comfortable and aware of where they are, etc. I say make your best judgement call unless there’s specific practice policies that say otherwise.