The Lost Apothecary by Sarah Penner

 Nella is an apothecary in London in the 1790s, following in her late mother's footsteps. But unlike her mother, some of Nella's potions are specifically intended to kill men who have done women wrong. One day Nella gets a note from a woman requesting a potion to take care of her husband, who keeps drugging and molesting the household help. The note is delivered by 12 year old Eliza, who is quite intrigued by the apothecary. Then Nella gets a request from Lady Clarence for a potion to kill her husband's mistress. Nella has a bad feeling about Lady Clarence, which turns out to be valid. 

At the same time as this story is unwinding, we meet Carolyn, a woman from Ohio who was supposed to be on a 10-year anniversary trip to London with her husband, except she found out that said husband had cheated on her right before they were supposed to leave, so she went to London without him. She finds a vial in the Thames, that turns out to have come from the apothecary's workshop, which sets her on a quest to find out more. The story keeps alternating between the 3 main characters--Nella, Eliza, and Carolyn.

It's a quick read and an interesting premise.