Rosemary Sutcliff wrote in the foreword to her “starkly simple” retelling of the story of Tristan and Iseult (OUP, 1971) about “one big change” she made in the story:
… In its far-back beginnings, Tristan is a Celtic legend, a tale woven by harpers around the peat fire in the timber halls of Irish or Welsh or Cornish chieftains, long before the time of chivalrous knights and fair ladies and turreted castles in which it is generally set.
The medieval troubadours took it and enriched it, and dressed it in beautiful medieval clothes, but if you look, you can still see the Celtic story, fiercer and darker, ad (despite the changes) more real, underneath. In this retelling I have tried to get back to the Celtic original as much as possible, and in doing this I have made one big change in the story.
In all the versions that we know, Tristan and Iseult fall in love because they accidentally drink together a love potion which was meant for Iseult and her husband King Marc on their wedding night. Now the story of Tristan and Iseult is basically the same as two other great Celtic love stories, Diarmid and Grania, and Deidre and the sons of Usna, and in neither of them is there any suggestion of a love potion. I am sure in my own mind that the medieval storytellers added it to make the excuse for being in love with each other when Iseult was married to somebody else. And for me, this turns something that was real and living and part of themselves into something artificial, the result of drinking a sort of magic drug.
So I have left out the love potion.
Because everybody else who has retold the tale in the past eight hundred years has kept it in, it is only fair to tell you this. I can only tell the story in the way which feels right to me in my own heart of hearts.