The Merck Manual of Diagnosis and Therapy, referred to as The Merck Manual, is the world's best-selling medical textbook, and the oldest continuously published English language medical textbook. First published in 1899, the current print edition of the book, the 20th Edition, was published in 2018. In 2014, Merck decided to move The Merck Manual to digital-only, online publication, available in both professional and consumer versions; this decision was reversed in 2017, with the publication of the 20th edition the following year. The Merck Manual of Diagnosis and Therapy is one of several medical textbooks, collectively known as The Merck Manuals, which are published by Merck Publishing, a subsidiary of the pharmaceutical company Merck Co., Inc. in the United States and Canada, and MSD (as The MSD Manuals) in other countries in the world. Merck also formerly published The Merck Index, An Encyclopedia of Chemicals, Drugs, and Biologicals. History and editions The first edition of The Merck Manual was published in 1899 by Merck & Co., Inc. for physicians and pharmacists and was titled Merck’s Manual of the Materia Medica. The 192 page book which sold for US $1.00, was divided into three sections, Part I (“Materia Medica”) was an alphabetical listing of all known compounds thought to be of therapeutic value with uses and doses; Part II (“Therapeutic Indications”) was an alphabetical compendium of symptoms, signs, and diseases with a list of all known treatments; and Part III (“Classification of Medicaments (sic) According to their Physiologic Actions”) was a listing of therapeutic agents according to their method of action or drug classification. Many of the terms used are now considered archaic, such as abasia, astasia, errhines and rubefacients - sternutatories, and many of the agents listed are now not considered to be standard therapeutic agents but were considered useful at the time, including poisonous compounds such as mercury, lead, strychnine and arsenic. There were 108 remedies listed for indigestion (dyspepsia), including alcohol, arsenic, cocaine, gold chloride, mercury, morphine, nux vomica, opium, silver nitrate, strychnine, and “Turkish baths (for malaise after dining out)”. Bismuth, calcium, magnesium salts were also on the list, which are ingredients found in many modern gastrointestinal treatments available today. Arsenic was recommended for over 100 illnesses including anemia, diarrhea, hydrophobia, elephantiasis, and impotence. The formulas include “aletris cordial”, a “uterine tonic and restorative”, which contained “aletris farinosa or True Unicorn combined with aromatics”. The manufacturer, Rio