Palabras clave: capacidad; consentimiento informado; decisiones de sustitución; principio de proporcionalidad.

Abstract: the right to informed consent has been qualified by the Spanish Constitutional Court as an act of exercising the right to physical integrity of the patient. This approach enables us to deal with two important issues related to “decide for others” in the medical context. First, the determination of the competence of the patient to consent by himself, recently considered by the Spanish Public Prosecutor’s Circular 1/2012. Second, the material criteria to guide the decisions adopted by the proxy. Taking into account the characterization of informed consent as a manifestation of a fundamental right, formalism in establishing competence and the uncritical and automatic transposition of the principle of autonomy into the decision-making process for incompetent people are overcome. On the contrary, it is stated the primacy of natural capacity and the use of the constitutional principle of proportionality as a tool to assess the legitimacy of the proxy informed consent.