Doctors & Dentists Mutual Privacy Agreements

It's becoming more common place for medical and dental professionals to ask patients to sign a mutual privacy agreement. The agreements from companies like Medical Justice are supposed to give the doctor or dentist ownership of any public commentary or review the patient might write in the future. Is this ethical? Do these agreements have any legal power?

Timothy Lee who writes about this in Ars Technica tells Scott Drake "censoring patients is the wrong way for doctors to deal with online criticism."