What does Gag clause mean in medical billing?

In managed healthcare, a gag clause would prevent health care providers from discussing all treatment options with their patients.

Medicare beneficiaries are entitled to advice from their physicians on medically necessary treatment options that may be appropriate for their condition or disease. Because a gag clause would have the effect of prohibiting a physician from giving a patient the full range of advice and counsel that is clinically appropriate, it would result in the managed care plan not providing all covered Medicare services to its enrollees, and therefore be in violation of the managed care plan's responsibilities.