Here is some of the info from amcas:
Only after May 15 are schools free to apply appropriate rules for dealing with accepted applicants who, without adequate explanation, hold one or more places in other schools. These rules should recognize the problems of the applicant who has multiple offers and also of those applicants who have not yet been accepted. Only schools whose first official day of classes begins prior to August 1 may start to request decisions from accepted applicants prior to May 15 but not earlier than April 15. This rule applies to all accepted applicants for the entering medical school class, including those individuals entering M.D./Ph.D. programs and individuals to whom merit scholarships or other special scholarships have been awarded.
By May 15 of the year of matriculation, an applicant who has received offers of admission from more than one school should choose the one school that he or she prefers and withdraw from all other schools to which he or she has been accepted.
*Prior to May 15 of the year of matriculation, an applicant should be given at least two weeks to reply to an offer of admission. After May 15, schools may require applicants to respond to acceptance offers in less than two weeks. An applicant may be required to file a statement of intent, a deposit, or both. The statement of intent should provide freedom to withdraw if the applicant is later accepted by a school that he or she prefers. Under no circumstances should an applicant be asked to withdraw from waiting lists as a condition of accepting a place.
*It is recommended that the acceptance deposit not exceed $100 and be refundable until May 15. After that date, a school may retain the deposit as a late withdrawal fee. If the applicant matriculates at the school, the school is encouraged to credit the deposit toward tuition.
Subsequent to June 1, a school of medicine seeking to admit an applicant already known to be accepted by another school for that entering class should advise that school of its intent. Because of the administrative problems involved in filling a place vacated just prior to the commencement of the academic year, schools should communicate fully with each other with respect to anticipated late roster changes in order to keep misunderstandings at a minimum.
After an applicant has enrolled in a U.S. school of medicine or begun a brief orientation program contiguous to enrollment, no further acceptances should be offered to that individual. Once enrolled in a school, students have an obligation to withdraw their applications promptly from all other schools. Enrollment is defined as being officially registered as a member of the first-year entering class at a school.
