"Informed consent" is when a patient agrees to go through with a medical procedure after having been completely informed by his or her doctor of all the relevant facts needed to make a decision. Those facts include risks, as well as benefits, possible outcomes, and alternatives. Additionally, you must be of sound mind in order to give informed consent — you cannot be mentally disabled, intoxicated, sedated (even mildly and medically), or severely sleep deprived. Almost always, informed consent must be written.
Serious ethical issues arise if a patient can’t give informed consent for any reason. Your doctor could be held liable for the harm or pain you suffered. If you feel your doctor withheld relevant information from you, or you were pressured into giving informed consent, you could have grounds for a medical malpractice lawsuit.