It is not unheard of but...... MOST medical schools see a BSN or nursing background and are NOT impressed. I have 2 friends that went this route. You are considered a "non-traditional student" (which means you will have to work twice as hard, and have better grades then the "traditional" candidates. Oddly enough you do not have to major in Biology or Chemistry, most prefer a Liberal Arts degree (with killer grades in upper level chemistry and physics, and shadowing a doctor or volunteer work). When my friend (a BSN with about 5 years experience, and a 4.0 science gpa) went on her interviews for medical schools. The FIRST question they always asked is "why are you considering a career change?". They dont see nursing as linked to medicine (very sad, but true). They also are very aware of the "nursing shortage" and do not want to take nurses from a field, that they feel is under staffed and needed. She was accepted to one in the end, but she had to work twice as hard.
I do know that DO schools prefer someone with a medical background of any kind. They can compete for the same residencies as the MD's, along with having a few just for DO's. A DO is in every way equivilent to an MD the training is just a little different (more holistic/focus on the total person). I am doing a BSN to MSN (ACNP) route myself, after heavy consideration of medical school. I think that since DO's have a more holistic view, if I were to have gone to medical school, it would have been the DO route.
Ask yourself "why am I drawn to nursing?" For me it was the patients, I like spending more time with them, educating them, and getting to know them a little better. That is why I decided to become a nurse practitioner, I still get to be a nurse, and have the autonomy I wanted.
GOOD LUCK and Best wishes whatever you choice is :)