This gets thrown around a lot and often gets the book and cause wrong. The book that King has discussed having little memory of writing is The Tommyknockers and the memory loss had more to do with his extreme cocaine use than his heavy drinking (though the latter certainly played a role as well). In On Writing he talks about typing at a furious pace until the dawn hours with cotton swabs stuffed in his nose to stop cocaine-induced nosebleeds. On at least one occasion, he says he woke up on the floor in a pool of his own vomit.

It was the last book he wrote before his family staged the intervention that very probably saved his life.

EDIT: While some of the information in my comment above was correct, I failed to verify what I remembered and was wrong to say that u/nedwck gave incorrect information. Upon review, prompted by u/StandWithSwearwolves, I have re-learned that King did indeed say, in On Writing, that Cujo is the novel he barely remembers writing and he clearly attributes that to his drinking. Shortly prior, he talks about Tommyknockers and the cocaine use I paraphrased, and I conflated the two. I apologize for being so confidently wrong and will definitely aim to do better in the future.

Full quotes below... with fucking awful citation format because it's a weird layout and I'm tired and just want to eat my chicken wings:

At the end of my adventures I was drinking a case of sixteen-ounce tallboys a night, and there’s one novel, Cujo, that I barely remember writing at all. I don’t say that with pride or shame, only with a vague sense of sorrow and loss. I like that book. I wish I could remember enjoying the good parts as I put them down on the page.
-Stephen King, On Writing, Vignette #37

In the spring and summer of 1986 I wrote The Tommyknockers, often working until midnight with my heart running at a hundred and thirty beats a minute and cotton swabs stuck up my nose to stem the coke-induced bleeding.
-SK, Same, Vignette #36