Physician-patient privilege
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In the laws of many common law jurisdictions, the concept of legal privilege, or the rule that certain conversations are so private and confidential that they cannot be used as evidence in court, extends to communication between a patient and physician. Although the rule is sometimes thought to apply only to situations such as admissions made to a psychiatrist during treatment, this is only one case.  In some jurisdictions, conversations between a patient and physician may be privileged in both criminal and civil courts.
The privilege may cover the situation where a patient confesses to a psychiatrist that he or she committed a particular crime.  It may also cover normal inquiries regarding matters such as injuries that may result in civil action.  For example, any defendant that the patient may be suing at the time cannot ask the doctor if the patient ever expressed the belief that his or her condition had improved.
The rationale behind the rule is that a level of trust must exist between a physician and the patient so that the physician can properly treat the patient.  If the patient were fearful of telling the truth to the physician because he or she believed the physician would report such behavior to the authorities, the treatment process could be rendered far more difficult, or the physician could make an incorrect diagnosis.
For example, a below-age of consent girl came to a doctor with a sexually transmitted disease.  The doctor is usually required to obtain a list of the patient's sexual contacts to inform them that they need treatment.  This is an important health concern.  However, the patient may be reluctant to divulge the names of her older sexual partners, for fear that they will be charged with statutory rape.  In some jurisdictions, the doctor cannot be forced to reveal the information revealed by his patient to anyone except to particular organisations, as specified by law, and they too are required to keep that information confidential.  If, in the case, the police become aware of such information, they are not allowed to use it in court as proof of the sexual conduct.

# United States
In the United States, the Federal Rules of Evidence do not recognize doctor-patient privilege.
At the state level, the extent of the privilege varies depending on the law of the applicable jurisdiction. For example, in Texas there is only a limited physician-patient privilege in criminal proceedings, and the privilege is limited in civil cases as well. See generally Texas Occupations Code section 159.003 and Texas Rules of Evidence, Rule 509(b).

# Confidentiality post Tarasoff
The case of Tarasoff v. Regents of the University of California in 1974 was an important precedent; it transformed Physician-patient privilege in many parts of the world. In this case the therapist was sued for failing to warn a third person of his patient's desire to harm her. Thus, for many therapists, harm to others will trump confidentiality and a therapist can and has been held liable for failing to inform appropriate people if their client reveals a risk to self or if the client makes a specific threat against a specific, named person.
Establishing the boundaries of confidentiality is an important part of any relationship with a therapist. Most usually this may need clarification in psychiatry, psychotherapy and other forms of counseling, but also in such areas as genetic counseling and infectious diseases medicine.