If you have begun a Masters program, you will be expected to have finished it before matriculating into medical school. Anythiing else is viewed as "poaching" from another program and it is considered bad form.
There are some exceptions made but overall I would expect most if not all medical schools to adhere to this unwritten rule in academia.
Most students do not earn a master's degree before proceeding from undergrad to medical school, if that's what you're asking. I know of no med school that requires students to earn a masters degree before matricuation.