The House on the Strand by Daphne du Maurier

When Dick Young's friend, Professor Magnus Lane, offers him an escape from his troubles in the form of a new drug, Dick finds himself transported to fourteenth-century Cornwall. There, in the manor of Tywardreath, the domain of Sir Henry Champerhoune, he witnesses intrigue, adultery and murder.

The more time Dick spends consumed in the past, the more he withdraws from the modern world. With each dose of the drug, his body and mind become addicted to this otherworld, and his attempts to change history bring terror to the present and put his own life in jeopardy.