First of all, do NOT forget to ask your interviewer if (s)he has a business card at the end of your interview. That should you give you snail mail & e-mail info. If not available, ask if (s)he has an e-mail address to which you might address any further correspondence or if you have any more questions.
Some ADCOM members will tell you to e-mail them if you have any questions (not a bad idea to think up one or two about 2 weeks after the interview, and before committee meets...just to bring your name to the forefront again). Obviously, if interviewed school is not high on your list of places to go, then don't worry about this extra bit.
But especially if your interviewer is over 50, (s)he will usually expect a thank-you note--what can I say--it's a generational thing! I suggest handwritten, on stationery. Go to your university book store & pick up a box of notecards with the university seal embossed on it (classy). Just something along the lines of, "Thank you for the time you spent with me on such-and-such date. I really enjoyed learning about the opportunities that Awesome Medical School provides and I hope to hear from you (positively) in the near future. Sincerely, Hopeful Pre-Med"
If something particularly interesting was discussed in the interview (fav sports team, your research, travel, etc.), another sentence referring to that will help jog the interviewer's memory.
If all you have is an e-mail address, then send an e-mail, but compose it formally & pay attention to your grammar & punctuation.
If students are on the interview committee & someone spent time with you, e-mail a thank-you there too. The more you do to stand out from everyone else is only a plus. I'm sure every aspect of you and your application will be discussed by the committee!
Hope this helps.