A term in medical ethics that derives from the ancient maxim primum non nocere, which, translated from the Latin, means first, do no harm.

The principle of nonmaleficence directs physicians to “do no harm” to patients. Physicians must refrain from providing ineffective treatments or acting with malice toward patients. This principle, however, offers little useful guidance to physicians since many beneficial therapies also have serious risks. The pertinent ethical issue is whether the benefits outweigh the burdens.
Physicians should not provide ineffective treatments to patients as these offer risk with no possibility of benefit and thus have a chance of harming patients.
In addition, physicians must not do anything that would purposely harm patients without the action being balanced by proportional benefit.

One of the most common ethical dilemmas arises in the balancing of beneficence and nonmaleficence.
This balance is the one between the benefits and risks of treatment and plays a role in nearly every medical decision, such as whether to order a particular test, medication, procedure, operation, or treatment. By providing informed consent, physicians give patients the information necessary to understand the scope and nature of the potential risks and benefits in order to make a decision. Ultimately it is the patient who assigns weight to the risks and benefits. Nonetheless, the potential benefits of any intervention must outweigh the risks in order for the action to be ethical.

UCSF School of Medicine. Ethic fast facts. Beneficence vs. nonmaleficence. Internet. Accessed on July 5, 2016.
UCSF School of Medicine. Ethic fast facts. The Principles of Medical Ethics: easy as A, B, C, D, E and F Internet. Accessed on July 5, 2016.