Are you sure it's by Roald Dahl? That description makes it sound oddly like "They Never Get Caught", but that's by Margery Allingham and is from 1936.
Harold Brownrigg, the villain of the piece, is a chemist and has the strange habit of lying down on his couch every Saturday evening and drinking brandy until he literally cannot move (not because anyone's poisoned him, just because he is so drunk).
I'll quote bits of a book review by Sammi Cox for the rest:
Harold Brownrigg is a chemist with a few money problems and a wandering eye. His wife, Millie, is just dull and stupid, nothing like some of the pretty girls that come into his shop. But none are so captivating as Phyllis, a young woman half his age.
However, Phyllis feels guilty about what they are doing even if he does not, and so decides to end it. This is too much for Harold, especially when he hears that Phyllis has been seen about town with a younger chap in a flashy car. It is becoming more and more obvious for Harold that Millie is just a problem he needs to get rid of. Once he has got rid of her, not only will it clear the way for him to be with Phyllis but he will also be able to get his hands on the money left to Millie by her father.
And so begins the intricate planning of a murder.
It's Harold who is planning to poison his wife.
Over-dark, round, hot eyes had Mr Brownrigg; not at all the sort of eyes for a little, plump, middle-aged chemist with a placid wife like Millie.
I felt very sorry for poor Millie as I moved through the story. Harold was extremely cold, even when he felt unnerved by what he was trying to do.
The twist in the tale – which I won’t mention – was very good. I didn’t expect it, probably because I was too preoccupied by the callous nature of Harold.
Of course, you know what the twist is.