At 18, information about your health, treatment, legal situation, education and other personal information generally cannot be shared with parents or others without your consent, unless you have a guardian appointed, have appointed someone to act on your behalf through a Power of Attorney, or designated someone as your healthcare surrogate,.  There is a federal law called the Health Information Portability and Accountability Act (HIPAA)[1] that protects the privacy of your health information.  It also creates a right to request amendments to your medical record, should you disagree with something in your records.

[1] The rules implementing this law can be found at 45 CFR Part 160 and Subparts A and E of Part 164.