Anna Margaretha Zwanziger (7 August 1760 – 17 September 1811) was a German serial killer. She used arsenic, which she referred to as "her truest friend". From 1801 until 1811, Zwanziger was employed as a housekeeper at the home of several judges in Germany. She would poison her employers with arsenic, and then nurse them back to health to gain their favour. She poisoned three people, and attempted to poison several others She killed four people, one of whom was a baby.