Hi Sonibun,
Your visa status will not have any effect on the admissions. The only thing that matters is your national board scores, written & pre-clinical exams (for some schools), GPA, work experience if any and your interview.
But you need to get the approval from INS before you start the school. This is a new INS rule, which is effective from 4/12/02. This rule prohibit persons admitted in either the B-1 or B-2 visitor category from attending school prior to INS approval of the request to change the status from visitor to either the M or F student category. Previously, applicants for a change of status could enroll in school upon filing for the change in status.
It is possible to apply to change status from tourist to student. Whether it would be approved or not depends on several factors, including how long one was here before applying for the change of status, what s/he told the consular officer when applying for the visitor's visa in the first place, and whether there is adequate financial support for school. An additional factor which is sometimes raised by USCIS, but more often by consulates, is whether or not one has shown an intent to return to the home country. An individual who enters on a B visa may not enroll and begin classes until after the change of status has been approved.
Also, if one travels abroad after the change of status approval, it would be necessary to obtain the F-1 visa at a consulate. It can be difficult to get the visa after having changed status, since the Consular Official would question why s/he did not obtain the F-1 visa originally, instead of entering as a visitor.
I hope this is helpful.
