The Apothecary’s Curse by Barbara Barnett

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. Five years later, hearingrumors of a Bedlam inmate with regenerative powers like his own, Simon is shocked to discover it’s Gaelan.