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, if a patient tells a physician he or she is feeling better, the defendant the patient may be suing at the time may not 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, let us say 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, it is likely that the girl might reveal her partners were older, and this could result in a fear that they would be charged with a crime such as statutory rape. In some jurisdictions, the doctor cannot be forced to reveal the information revealed by his patient to anyone except the appropriate health authorities, 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.

In the United States, the extent of the privilege varies depending on the law of the applicable state. For example, in Texas there is virtually no meaningful 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).