The Apothecary's Curse — Barbara Barnett

"The Apothecary's Curse" is an alchemist's brew of historical fantasy and cutting edge genetics as a physician Simon Bell and apothecary Gaelan Erceldoune race against time to recover the ancient book that made them immortal centuries ago. 

In Victorian London, the fates of physician Simon Bell and apothecary Gaelan Erceldoune entwine when Simon gives his wife an elixir created by Gaelan from an ancient manuscript. Meant to cure her cancer, it kills her. Suicidal, Simon swallows the remainder--only to find he cannot die. Years later, hearing rumors of a Bedlam inmate with regenerative powers like his own, Simon is shocked to discover it's Gaelan. 

Two centuries later, a modern-day pharmaceutical company unearths diaries describing the torture of Bedlam inmates, and the company's scientists suspect a link between Gaelan and an unnamed inmate. But when Gaelan and Transdiff Genomics geneticist Anne Shawe are powerfully drawn to each other, her family connection to his manuscript leads to a stunning revelation. Will it bring ruin or redemption?