I'd be REALLY careful about that--it could seriously backfire, as most advisors aren't going to want to pour time and resources into a student if they explicitly know there's a good probability that the student will "jump ship" into another profession in a year or two, especially if that would mean not completing the PhD program.
Having an incomplete PhD isn't an option. As discussed ad nauseam on the "nontraditional" forum, med schools are extremely unlikely to accept you if you've quit a previous program. Quitting isn't an option. If you can leave with a masters degree that might be okay, but leaving with nothing makes you look like a quitter and few medical schools are going to take that risk.
futureapppsy2 has a point about telling the advisor. It's a double-edged sword. The advisor might not like the idea. Some advisors want nothing less than to create clones of themselves, and if you don't look like you're going to be a clone then you could be digging your own grave. It's possible that they might try to withdraw some of their resources, but if they've accepted you as a student then the school shouldn't allow them to force you out unless you've engaged in real misconduct or have been put on academic probation. Keep in mind, too, that grad schools don't like to have bad degree-completion rates (this gets reported during accreditation, and with a small sample of students even one non-completer can make a big difference on their percentages).
So your advisor can make things difficult for you if they don't like the plan.
But you're not going to get into med school without your advisor's help, even if you wait until after grad school to take your pre-reqs, so you have to tell your advisor at some point.
As mentioned above, leaving without the PhD isn't an option. But if you apply to medical school with a PhD and you don't have a letter of rec from your PhD advisor, that is also very likely to sink you before you even get an interview. The admissions committee is going to say "This person worked for 5+ years with someone as their personal career mentor and work supervisor, and they didn't write a letter from that person?! Must be hiding something. On to the next."
And even if you plan on taking pre-med pre-reqs after finishing your PhD, your little plans and schemes are going to reveal themselves before you graduate. Your advisor is going to ask you about internships or post-doc positions, or faculty positions, or whatever else they might think about your career plans (sorry, I'm not positive whether you're a clinical psych PhD student or experimental of some sort). And you won't be able to shrug your shoulders and say "I'm waiting to figure it out until after I graduate."
The best advice I can offer as someone who's been in a similar situation is that you are honest, open, and not demanding or entitled. Reassure your advisor that you plan to finish your PhD, that you love what you're doing, but that you've been coming to the conclusion that your heart lies in a different field. Tell them that you want to finish your PhD because you are personally committed to the project--it's important to you and you want to see it finished.
Tell them that you're committed to being an excellent student throughout your PhD work, and that you're not going to let anything compromise that. But if you're serious about med school, you're going to need those classes, so the clock is ticking. Especially if you're missing your chemistry courses. Most schools require a full year of general chemistry and a full year of organic, plus possibly some amount of biochemistry. If a school wants all of these, it's going to take you 2+ years to get them all finished, so you'll want to start ASAP.
If your advisor cares about you (or is just a decent human being) then they aren't going to treat you differently just because you don't want to be just like them. Their job as an advisor is to help you find your own path... not all advisors realize this, unfortunately, but if you talk to enough professors you'll find that many do feel this way.
If you think that your advisor is going to be the type of person who does not like this news, then you're going to have to ease them into it. If you're a clinical psychology student, you might start by talking about how excited you are about prescription privileges for psychologists. Start bringing up health psychology articles in conversation. In other words, start showing that you're interested in medicine.
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Also, I don't know if we're putting the cart before the horse here. You've said you're toying with the idea of pre-med classes. But how sure are you that medicine is right for you? Have you done much physician shadowing? (Or any?) Any medical volunteer work? You should definitely be pretty darn committed to the whole shabang before you break it to your mentor. (And before you start paying money for classes!)