Green card holders (non-US citizen but permanent residents) hold the same weight as a US citizen and are eligible to apply to US Medical schools. However, international students with student visas and without permanent residency status are not eligible to apply to some medical schools.
The National Association of Advisors for the Health Professions has put together a list of schools with their policies on accepting international students.
MEDICAL SCHOOL ADMISSION POLICIES TOWARDS NON-U.S. CITIZENS (external link)