The doctrine of informed consent is a relatively new doctrine, arising from our horror from many medical experiments in which people were subjected to treatment or lack of treatment without any understanding or knowledge of what was happening to them. The Tuskegee Study and HELA replication are only two examples in recent history.
The informed consent doctrine is used by means of a consent form in all states, I believe. The patient, or in the case of a study, the subject, is presented with a paper that outlines the medical procedure or treatment, including all the possible risks. The patient is supposed to be given sufficient time to read this and to ask questions before signing it, but in many instances, people just sign, not really understanding what is involved.
The whole point of the doctrine, from the patient's perspective, is that our bodies are our own, and that any treatment should be our decision, understanding what the treatment is and what the risks are.
From the doctor's perspective, informed consent acts as a protective device, in the event of an accusation or a lawsuit later on. If things go wrong, the doctor is able to establish that the patient knew going in that there were risks.