Female university lecturer drinks potion and enters another dimension - as a mouse/rat? [s]

[Mimsy]

This is a novel for adults published about four or five years ago I think. This woman, a junior lecturer at an English university, gets hold of a book which tells her the secret of slipping into some other dimension. She has to make some sort of potion and drink it...then she passes out and her subconscious goes into this other world...there are mice or rats there...I think perhaps she becomes a mouse/rat. I think the potion might have charcoal in it. And I think she was having a rather self-destructive affair with an older man at the beginning of the book. That's all I can remember!

[Kagama-the Literaturevixen]

The End of Mr. Y?

[Mimsy]

YES!!! Thank you so much!