SOLVED. main character takes an unknown drug, time travel [s]

[Naomi]

the main character was staying in a friends house. He discovers this drug his friend was taking? One to take him somewhere and one to bring him back. The main character experiments with it over the course of his stay and believes hes traveling back in time to the very place hes staying just hundreds of years prior. He takes the drug and it almost transports him. He can watch what was happening/happened while on the drug. I don't remember much other than that. I do remember a character in the book was named Isolde, but this certainly was not any book about Tristan and Isolde. Any help would be much appreciated. I've been searching for this book for over 10 years and don't know what else to do.

[Shonari]

Maybe The House on the Strand

[Lobstergirl, au gratin]

I second The House on the Strand - he goes back to medieval times and meets a Lady Isolda.

[Naomi]

Yes thank you! This is it. Wow I've been looking for this for years. Huge help guys really thank you!