I implicitly trust most of what I hear from a doctor, even when it defies intuition. But I have much less trust for what I hear secondhand, i.e. when someone says "my doctor said". There's simply too great a likelihood for signal corruption along the way. Patients can fail to communicate context to their doctors, miss subtleties in the communication back, and fail to relay context once again when they relate their stories to other lay folk. And no, I am not a doctor :)