Depends on what your definition of OK is.
If you mean does it ‘break’ a fast and stop Autophagy. No it doesn’t.
If you mean does it ‘break’ a fast as per ramadan standards – then yes it does.
Will it help ease a water fast – possible, it is an appetite suppressant, I certainly started mine doing this – however you also have Black Coffee and Apple Cider Vinegar to also help suppress your appetitive – and you will find after day 2-3 any remaining hunger pains will go away.
Quiting smoking is a whole other topic, and I won’t go lecture you regarding its toxicity to the system.
I actually did quit during my 10+ day water fast on day 4.
The idea of fasting and healing myself at exponential levels whilst also taking in one of the most toxic substances on the planet just didn’t seem right.
Also by day 4 the smoke was quite a strong lingering smell and when you are just drinking water and got ash in your mouth its not a pleasent combination. I moved across to vaping which is a whole other topic. But you may find it unpleasent over time and give you a good reason to just pause it.
I’ll leave you with a quote that I quite liked that helped me to stop at that time.
“Smokers smoke so that they can feel what its like to be a non smoker”
Really makes you question what you are doing.
In Summary: No it doesn’t break you fast (unless its for ramadan).
However: It’s going to hinder your healing process – how much is unknown but intuition says a lot.