I would hence to figure that most of the time your doc won't call the cops on you if you told him/her you use MDMA or other substance. Of course, this depends on the doctor, and as mulberryman pointed out in the above posting, some docs in these small hick towns and other conservative hamlets (in the south or midwest, for example) might think it would be "proper" to get the law involved. Out here in California, I have found that most of my friends who are docs are pretty open minded about such things.
In medical school, physicians usually have ethics training, and rest assured, this subject is brought up and discussed. It's clearly controversial and debatable. As many others have pointed out here, the most troubling aspect of admitting to your physician that you use drugs is that they will most always write it down. They do this for many reasons. Among them are:
1. They were trained to.
2. They are likely required to by law.
3. For liability/malpractice reasons. If you are discussing a pre-existing condition, or other drugs which may/will cause conflict with potentially prescribed meds, for example.
4. Insurance companies require records.
5. DEA/FDA requires it if a prescription is being written.
Doctors will write down what you tell them, period. If you do not want this info connected with you in any way, you are best advised to visit a new doctor, forgo using any insurance, and NOT using your real name, address, SSN, or other personal info.
Of course, failing to disclose medical conditions or drugs being taken on a questionnaire can be dangerous. It's possible to be 100% honest on this paperwork, but just don't use your real identifying info on any other forms, especially anything having to do with insurance.