Dr Su,
Have you been accepted to med school already? or are you planning on applying to med school in the future and are intending to enroll in the masters prog in the interim?
In general, MD programs seriously frown upon applicants who intend to abandon their current programs; be it a PhD, Masters, PharmD or whatever. It shows a lack of committment and lack of loyalty. Withdrawing not only throws the entire program off (they can't replace you mid-cycle), but also means that all the $ invested in you by that prog that far is wasted. A friend of mine with 41 MCAT had to apply three times. He claims he only got in on the 3rd attempt because he finally completed the PhD program he was planning on withdrawing from for med school. i.e you run the risk of being avoided (or even black-labelled) by med schools
If you've already been accepted to med school, then much of the above doesn't apply to you. However you then owe it to your conscience to defer med school and finish the program. That way you also don't initiate a life pattern of abandoning work half-way through.
I appologize for the preaching. Didn't mean to sound condescending or anything. Just trying to answer your Q. Hope this helps. Good luck.