Informed consent is part of medical ethics. It involves having a patient or research participant, or their guardian or surrogate, assess the risks and benefits of the proposed treatment or experiment, and give consent only on agreeing that the potential benefits outweigh the risk. Not all clinical research will provide a direct benefit to the subject, but the benefits must be seen as to society.
Worldwide, the best accepted statement of medical ethics, including informed consent, is the Declaration of Helsinki, last revised in October 2008. The Declaration is maintained by the World Medical Association (WMO). The WMO, not to be confused with the World Health Organization, is a nongovernmental organization of voluntary national medical associations. 
At the time of the 2000 revision,  the "most contentious issues included:
What has easily been the most contentious issue is the standard of treatment that should be assured to human participants involved in research projects. The WMA has affirmed its position that at the conclusion of any research study, every patient entered into a research project should be assured of the best proven prophylactic, diagnostic and therapeutic methods identified by that study". "The WMA has also been very concerned about the possible exploitation of so-called "poor" nations for research by companies from so-called "rich" countries. New guidelines in the Declaration will now call on researchers to make sure that there is a reasonable possibility that the local participants in a study, including their fellow countrymen, would be able to benefit from the results (whether positive or negative) of such studies". — Dr. Delon Human, Secretary-General, World Medical Association
Shortly after the Second World War, it became obvious that Nazi physicians had performed nonconsensual, hazardous, and even fatal Nazi experiments on human beings, without obtaining their consent in any way. An immediate need was perceived to develop rules that would present such atrocities, for which a number of the experimenters were tried, and some executed. The 1947 Nuremberg Code was a basic set of principles from that Trial, which evolved into the Declaration of Helsinki.
It was later found that Japan had also conducted similar lethal research.. Western democracies, however, were not free of such actions, such as the Tuskegee Syphilis Experiment in the United States, and U.S. experiments during the Cold War, such as the CIA MKULTRA research program on interrogation and "mind control."
Revisions to the Declaration of Helsinki are being considered.
The conditions, risks, and benefits of the proposed treatment or experiment must ethically be explained to the patient in terms he or she is reasonably expected to understand. This is usually interpreted as having a nontechnical summary presented, by a health care worker, who is able to answer questions or find the answers.
In some cases, especially with increased information availability, the informed consent document may present a different problem than not being understandable to a layman: it may not contain enough data to let a knowledgeable individual make an informed decision. It is not, however, established, that a participant in a research trial, for example, has a right to see the full medical description of the proposed research.
Use of placebo
A placebo is a drug or treatment that has no therapeutic effect, given to a patient either as a psychological suggestion in treatment, or as to rule out psychological effects of the act during research. The use of placebos in randomized controlled trials remains controversial. Current policy is defined by Article 32: "The benefits, risks, burdens and effectiveness of a new intervention must be tested against those of the best current proven intervention, except in the following circumstances:
- The use of placebo, or no treatment, is acceptable in studies where no current proven intervention exists; or
- Where for compelling and scientifically sound methodological reasons the use of placebo is necessary to determine the efficacy or safety of an intervention and the patients who receive placebo or no treatment will not be subject to any risk of serious or irreversible harm. Extreme care must be taken to avoid abuse of this option."
The use of placebos remain controversial. One argument for their use is to validate whether the accepted treatment is indeed beneficial. If the use of a placebo is otherwise ethical, the patients or subjects must be protected from pain, psychological stress, or other adverse effects.
Situations where consent could not be obtained
If a treatment being tested is for emergency situations where a patient would be likely to be unconscious and nearly dead, and thus unable even to be informed, much less consent, is it possible to have trials? Article 26 addresses this with
Research on individuals from whom it is not possible to obtain consent, including proxy or advance consent, should be done only if the physical/mental condition that prevents obtaining informed consent is a necessary characteristic of the research population. The specific reasons for involving research subjects with a condition that renders them unable to give informed consent should be stated in the experimental protocol for consideration and approval of the review committee. The protocol should state that consent to remain in the research should be obtained as soon as possible from the individual or a legally authorized surrogate.
It would seem relatively straightforward, for example, that a parent or other guardian can give informed consent on behalf of a child. On further examination, this is not so clear, and advancing technology makes it more so.
Could, for example, a parent ethically expose one child to the risks of being an organ donor to another child? Assume that the child is too young to make an independent decision.
In some real-world situations where parents had an additional child to provide a "spare part" to an existing child with a treatable illness, is it ethical for health care workers to participate in subjecting the second child to some risk?
Access to treatment
One challenge, which has been especially visible in trials of Human Immunodeficiency Virus (HIV) treatments in the Third World, is whether study participants can be enrolled only if they receive, if in a control group, the "gold standard" treatment available in the industrialized world. Article 30 of the current document states,
At the conclusion of the study, every patient entered into the study should be assured of access to the best proven prophylactic, diagnostic and therapeutic methods identified by the study.
A footnote adds,
The WMA hereby reaffirms its position that it is necessary during the study planning process to identify post-trial access by study participants to prophylactic, diagnostic and therapeutic procedures identified as beneficial in the study or access to other appropriate care. Post-trial access arrangements or other care must be described in the study protocol so the ethical review committee may consider such arrangements during its review.
There is an ethical challenge if it is economically and logistically impractical to provide a first-world standard of care to participants in a third-world trial, either during the trial or in post-trial care. The argument is made that even a less-than-industrialized-world standard may be better than the care they could otherwise receive. Insistence on first-world standards might mean there is no access to improved care, but this can be seen as exploiting economically deprived societies. 
If, in a treatment situations, the clinician(s) involved, with appropriate consultation, determines that any further active (i.e., disease-modifying rather than comfort-increasing) treatment is futile, can the patient or surrogate insist on that treatment?
Nature of consentArticle 22 states,
In any research on human beings, each potential subject must be adequately informed of the aims, methods, sources of funding, any possible conflicts of interest, institutional affiliations of the researcher, the anticipated benefits and potential risks of the study and the discomfort it may entail. The subject should be informed of the right to abstain from participation in the study or to withdraw consent to participate at any time without reprisal. After ensuring that the subject has understood the information, the physician should then obtain the subject's freely-given informed consent, preferably in writing. If the consent cannot be obtained in writing, the non-written consent must be formally documented and witnessed.
It has been proposed to allow a waiver "when the research involves only slight risk or when the procedures to be used are customarily used in the practice of medicine without documentation of consent". An argument for this is that it reduces time and effort to document consent, potentially increasing access to new therapies, but the argument against is that it substitutes the clinician's judgment of the risk and benefit to the patient. When the consent is for research, the researcher may have a conflict of interest between the needs of the experiment and the protection of the patient. 
- World Medical Association (adopted 1964, with amendments through 59th WMA General Assembly, Seoul, October 2008), Declaration of Helsinki: Ethical Principles for Medical Research Involving Human Subjects
- World Medical Association, Policy
- World Medical Association (9 October 2000), Press release: WMA Revises The Declaration Of Helsinki
- Nuremberg Military Tribunal: the Doctors' Trial
- The Nuremberg Code
- Williams, Peter & David Wallace (1989), Unit 731: Japan's Secret Biological Warfare in World War II, The Free Press
- Stockhausen, Kate (2000), "The Declaration of Helsinki : revising ethical research guidelines for the 21st century", Medical Journal of Australia 172: 252-253