Public medical schools are supported by state funding that also supports bringing residents of the state into these medical schools. Different schools have different requirements in terms of how many state residents they must accept. The Association of American Medical Colleges reports on how many students apply and are accepted within state in their FACTS data reports. 

Table A-1 shows the in-state/out-of-state resident ratios for each MD school. Table A-5 shows how many applicants come from each state, and what ratio matriculate into medical school.
 
If you’re from a state with a higher ratio of seats to applicants, it does increase your chances of admissions given supply and demand. In 2021, 14.9% of the 7,900+ California residents who applied were able to matriculate to an in-state medical school; by contrast, 49.6% of the 256 West Virginia applicants matriculated in state.

But, there are plenty of private schools with no state residency preference, so there are still opportunities for all applicants even if your state school seats are scarce or you don’t have a state school at all.

It depends on which medical schools you’re talking about. U.S. allopathic and osteopathic medical schools are either private or public. In most cases, private schools do not prefer applicants from certain states. Public schools, however, are mandated by the state to allocate a certain percentage of seats to in-state residents (in exchange for state financial support). Many public schools also have missions specific to serving the residents of their state, and so prefer students with a commitment to the state.

Many public medical schools – with class sizes between 100 and 200 – have fewer than 10 entering students who are out-of-state. If you are out-of-state, the odds are so low that your time and money are likely better spent applying elsewhere.

But there are other state schools (Michigan and Virginia for example) that matriculate more non-residents than most state medical schools.

If you’re curious about a public medical school and you’re not from that state, consult the AAMC Medical School Admission Requirements (yearly subscription, or access the resource for free at HPA). The MSAR lists the in/out of state ratios of applicants, interviewees, and matriculants.

This can vary from state to state. Generally, if a parent/guardian claimed you as a dependent on their tax return, and they reside in your "home" state, you can maintain that state for your state residency. Maintain your driver's license and voter registration in this home state; these are "markers" that are frequently used to classify your state residency. If you change some of these markers (e.g., independent/dependent status, license, voting) to another state, things are more likely to become complicated. If you're ever in doubt about how a change of locale might affect your in-state status with the public medical school in a particular state, call the Admissions office and they will direct you to someone who can help.

California is one of the tougher states in which to gain residency. You would not be considered a California resident until after you had worked (not attended school) in California for a year (see policy here). You would have to get a California driver’s license and register to vote at the very beginning of that first year, too. Thus, you could not call yourself a California resident while applying even if you were living and working in the state. Your odds of acceptance at a state medical school would improve if you spent a year working and establishing residency and then applied as a California resident. While the California schools are an attractive option because of the price, there are more Californians applying to medical school than residents of any other state and not that many med school spots to go around. The competition would be very tough even if you became a “real” resident. This is a long way of saying that it might be wiser to retain your residency in your current state if you can do so.

The first thing is, in terms of residency rules, states differ.  In fact, the residency rules for public medical schools sometimes vary within the same state!  Your first step when considering a change in residency is to contact a residency official at the institutions in question.  Sometimes these officers are found in the Registrar's office, sometimes in Admissions, sometimes in a "residency" office all by themselves; usually the medical schools Admissions office can direct you.

Other things to remember:

• You may only list one state of residence on your AMCAS application.
• If you are a dependent on either parent's/guardian's tax return, then you are a resident of their state. If you are considering switching states and you're still enrolled as a college student, then most likely it is because one or both parents are moving.  If this is the case, make sure you change your driver's license and voter registration to match your parents' address.
• For those who have graduated at the time they apply to medical school—and are claiming residency in a new state, separate from their parents’—they should have a permanent address, driver's license, and voter registration card in the new state.
• The Medical School Admissions Requirements, or "MSAR," includes the numbers of "resident" and "non-resident" applicants, interviewees, and matriculants.  Sometimes you may increase your chances of admission to a medical school when you're lucky enough to be able to declare residency in a state with a good ratio of public medical school seats to state residents.