“Doctors pour drugs of which they know little, to cure diseases of which they know less, into human beings of whom they know nothing” was credited to the French writer and philosopher Voltaire (1694-1770) in 1955. The exact quotation is not known, but earlier versions differed slightly from this.
 
The earliest known version was cited in a 1787 newspaper:
 
“Voltaire had not much reverence for physicians, or any of their auxiliaries; he said of a young apothecary, who accused him of having spoken too freely of the order, that an apothecary was of no use but to pour drugs, of which he knew little, into a body, of which he knew less.”
   
“VOLTAIRE Once said of an apothecary, that his employment was to pour drugs, into which he knew little, into a body of which he knew less” was cited in a newspaper in 1818. “Doctor.—According to Voltaire, one whose business it is to pour drugs, of which he knows little, into a body of which he knows less” was cited in 1824.
 
     
Wikipedia: Voltaire
François-Marie Arouet (French: [fʁɑ̃.swa ma.ʁi aʁ.wɛ]; 21 November 1694 – 30 May 1778), known by his nom de plume Voltaire (/voʊlˈtɛər/; French: [vɔl.tɛːʁ]), was a French Enlightenment writer, historian, and philosopher famous for his wit, his attacks on the established Catholic Church, and his advocacy of freedom of religion, freedom of expression, and separation of church and state. Voltaire was a versatile writer, producing works in almost every literary form, including plays, poems, novels, essays, and historical and scientific works. He wrote more than 20,000 letters and more than 2,000 books and pamphlets. He was an outspoken advocate, despite the risk this placed him in under the strict censorship laws of the time. As a satirical polemicist, he frequently made use of his works to criticize intolerance, religious dogma, and the French institutions of his day.
 
Wikiquote: Talk:Voltaire
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Doctors are men who pour drugs of which they know little, to cure diseases of which they know less, into human beings of whom they know nothing.
       
8 November 1787, Columbian Herald (Charleston, SC), “Extract of a letter from Bengal,” pg. 2, col. 3:
Voltaire had not much reverence for physicians, or any of their auxiliaries; he said of a young apothecary, who accused him of having spoken too freely of the order, that an apothecary was of no use but to pour drugs, of which he knew little, into a body, of which he knew less.
 
27 August 1818, Berkshire Star (Stockbridge, MA), pg. 1, col. 3:
VOLTAIRE
Once said of an apothecary, that his employment was to pour drugs, into which he knew little, into a body of which he knew less.
 
1 November 1824, London Morning Post (London, Middlesex), pg. ?:
Doctor.—According to Voltaire, one whose business it is to pour drugs, of which he knows little, into a body of which he knows less.
 
23 May 1829, The Portsmouth Journal and Rockingham Gazette (Portsmouth, NH), pg. 1, col. 3:
This class of physicians (quacks—ed.) practise the grossest imposition with impunity; they pour drugs of which they know little, into bodies of which they know less.
(...)
May 14, 1829.
CINCINNATUS.
 
Google Books
An Exposure of the Causes of the Present Deteriorated Condition of Health
By Joel Pinney
London: Longman, Rees, Orme, Brown, and Green
1830
Pg. 163:
How is it, then, that men should be so weak as to pour drugs, of the properties of which they know little, into their bodies — of the nature of which they know less— upon the faith of some ignorant presuming quack?
 
6 January 1831, The Star (London), pg. 4, col. 4:
AN APOTHECARY’S KNOWLEDGE. Voltaire said of an apothecary, that his employment was to pour drugs, of which he knew little, into a body of which he knew less.
 
1 February 1832, The National Aegis & General Advertiser (Worcester, MA), “Modern Meanings of Old Words,” pg. 1, col. 3:
Doctor. One whose business it is to pour drugs of which he knows little, into a body of which he knows less.
 
Google Books
5 December 1832, New England Farmer, pg. 168, col. 2:
Quacks and self-doctorers often “pour drugs” of which they know little, into bodies of which they know less.
 
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Gale Middleton:
A Story of the Present Day
By Horace Smith
London: Richard Bentley
1833
Pg. 44:
“Why couldn’t'st be a doctor or a surgeon?”
 
“To be a butcher of human carcasses, a dissector of dead bodies and a tormentor of living ones; to be conversant with misery, anguish, and putrifying sores; to pour drugs of which I know little, into a frame of which I know less; to see none but sufferers,...?”
 
Chronicling America
25 March 1840, The North Carolina Standard (Raleigh, NC), pg. 2, col. 2:
Like every wise physician, he has much confidence in the vis medicatrae naturae—the medical power of nature, doubting at the same time whether diseases are often cured by men “who pour drugs, of which they know little, into stomachs of which they know nothing.”
 
Chronicling America
14 June 1855, Fayetteville (TN) Observer, pg. 4, col. 3:
Voltaire said of an apothecary, that his employment was to pour drugs of which he knew little, into a body of which he knew less.
 
Chronicling America
13 April 1879, Juniata Sentinel and Republican (Mifflintown, Juniata County, PA), pg. 4, col. 2:
A Great French Philosopher
once defined a doctor to be “a person who pours drugs about which he knows little, into a body concerning which he knows less, in order to cure diseases of which he knows nothing.”
 
3 March 1881, Cleveland (OH) Leader, “Meetings of the Alumni of the Cleveland and Wooster Medical Colleges,” pg. 3, col. 2:
Voltaire sarcastically said “a man who will pour drugs of which he knows nothing, into a body of which he knows less, is an ignorant man, a good physician never.”
 
18 December 1881, New York (NY) Herald, “Chat by the Way,” pg. 6, col. 3:
Voltaire, who is not always to be trusted, spoke of a physician as “a man who pours drugs of which he knows little into bodies of which he knows less.”
 
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October 1889, The New Jersey Law Journal, pg. 323:
Maybrick trial quoted Voltaire’s epigrammatic description of doctors, as persons who ‘“pour drugs of which they know little into stomachs of which they know less.”
 
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15 February 1893, Australian Medical Journal, pg. 79:
The oft-quoted taunt that we are people who pour drugs of which we know little, down the throats of patients of whom we know less, no longer makes us smart by its truthfulness.
 
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June 1899, Eclectic Magazine, pg. 927:
There was sadly too much truth in Voltaire’s bitter sneer, “Doctors pour drugs of which they know little, into bodies of which they know less,” and I fear the sting has not entirely gone out of it even in this day of grace.
 
Google Books
Speaker’s Handbook of Epigrams and Witticisms
By Herbert V. Prochnow
New York, NY: Harper
1955
Pg. 82:
Doctors pour drugs of which they know little, to cure diseases of which they know less, into human beings of whom they know nothing.
Voltaire
 
Google Books
Words of Wellness:
A Treasury of Quotations for Well-Being
Edited by Joseph Sutton
Carson, CA: Hay House, Inc.
1991
Pg. ?:
Doctors pour drugs of which they know little, to cure diseases of which they know less, into human beings of whom they know nothing. —Voltaire
 
Google Books
Medical Jokes and Humour
By Clifford Sawhney
New Delhi: V & S Publishers
2011
Pg. 143:
Voltaire (1694-1778): “Doctors pour drugs of which they know little, to cure diseases of which they know less, into human beings of whom they know nothing.”