Primum non nocere
Primum non nocere is a Latin phrase that means "First, do no harm."  The phrase is sometimes recorded as primum nil nocere.
It is one of the principal precepts all medical students are taught in medical school. It reminds a physician that he or she must consider the possible harm that any intervention might do. It is most often mentioned when debating use of an intervention with an obvious chance of harm but a less certain chance of benefit.
Since at least 1860, the phrase has been a hallowed expression for physicians of hope, intention, humility, and recognition that human acts with good intentions may have unwanted consequences.

# Origin
The origin of the phrase is not widely known; contrary to popular belief, the phrase is not in the Hippocratic Oath. However, it is often described as a Latin paraphrase by Galen of a Hippocratic aphorism (despite the fact that Galen also wrote in Greek rather than Latin); yet no specific mention in Galen's writings has been reported. The closest approximation to the phrase that can be found in the Hippocratic Corpus is "to help, or at least to do no harm," taken from Epidemics, Bk. I, Sect. V.
According to Gonzalo Herranz, Professor of Medical Ethics at the University of Navarre, Primum non nocere was introduced into American and British medical culture by Worthington Hooker in his 1847 book, Physician and Patient. Hooker attributed it to the Parisian pathologist and clinician Auguste Francois Chomel (17881858), the successor of Laennec in the chair of medical pathology, and the preceptor of Pierre Louis. Apparently, the axiom was part of Chomel's oral teaching. :"The origin of primum non nocere." British Medical Journal electronic responses and commentary, 1 September 2002.
However, close examination reveals that Hooker did not use the specific expression or the traditional Latin phrase. A detailed investigation of the origins of the aphorism was reported by the clinical pharmacologist Cedric M. Smith in the April 2005 issue of the  Journal of Clinical Pharmacology.  It addresses the questions of the origin and chronology of appearance of the maxim. Rather than being of ancient origin as usually assumed, the specific expression, and its even more unique associated Latin phrase, has been traced back to an attribution to Thomas Sydenham (16241689) in a book by T. Inman (1860). The book by Inman, and his attribution, was reviewed by "H.H." in the American Journal of Medical Science in the same year.  A prominent American surgeon, L.A. Stimson, used the expression in 1879 and again in 1906 (in the same journal).  That it was in common use by the turn of the century is apparent from later mentions, such as by the prominent obstretician J. Whitridge Williams in 1911, as well as detailed discussion of its use in a popular book authored by Dr. Morris Fishbein, the long-time editor of the Journal of the American Medical Association in 1930.
The article also reviews the various uses of the now popular aphorism, its limitations as a moral injunction, as well as the increasing frequency of its use not only in medical but other contexts as well.

# Primum succerrere
With regards to cancer therapy, the dictum primum non nocere is not necessarily the guiding principle. "The goal of cancer treatment is first to eradicate the cancer. If this primary goal cannot be accomplished, the goal of cancer treatment shifts to palliation, the amelioration of symptoms, and preservation of quality of life while striving to extend life." Primum non nocere would therefore, theoretically, give way to primum succerrere - "first, hasten to help." When cure of cancer is possible, cancer treatments may be undertaken despite the certainty of severe and perhaps life-threatening toxicities. Every cancer treatment has the potential to cause harm, and treatment may be given that produces toxicity with no benefit. The therapeutic index of many interventions is quite narrow, and most treatments are given to the point of toxicity. Conversely, when the clinical goal is palliation, careful attention to minimizing the toxicity of potentially toxic treatments becomes a significant goal.