I started in science with dual science undergraduate degrees, worked in that field for a number of years, and then decided to go to law school to become an environmental lawyer. I practiced at a large NYC law firm, then I went to work for a large company. At the company I've practiced many different types of law (starting with environmental and evolving from there).
If you are truly passionate about both subjects, you can do both. Healthcare law, intellectual property/patent law, medical malpractice law and other specialties are a great combination of all your interests -- if those are still your interest 7 years from now! (It usually takes 4 years for undergraduate degree, and 3 years for law school.)
If you want to be a lawyer, you're going to need really strong grades at university - so if science is your thing, and you'll be able to maintain a good Grade Point Average (GPA), choose a science major for your undergraduate degree. You don't need to have any specific undergrad degree to attend law school. If law school is of interest to you, be sure to add some electives classes to your university class that will balance out the science and reflect more traditional law school pathways - writing composition, political science, history, etc. To excel in law school you'll need logic, reasoning and strong writing skills -- and science certainly fosters the first two! You'll have to be sure that you develop the writing skills on your own through elective courses, because most science majors aren't required to work hard on their writing.
If you are still focused on medical science vs lawyer, here are some considerations:
(a) education requirements - you can be a scientist with a 4 year bachelor's degree (4 years minimum); however, you'll get more senior positions and perhaps more control over your work in science if you continue on to get a masters and/or PhD. At minimum to be a lawyer, you'll need both a 4 year bachelor's degree and a 3 year juris doctorate (JD) degree (7 years minimum). Also, you need to pass the Bar Exam.
(b) education expense - it cost a lot to pay for both undergraduate and graduate school and/or law school. This often translates to student loans...
(c) salary - a top lawyer at a top firm in a big city will typically make a lot of money; however, that describes about 1% of all lawyers. Most lawyers work in companies, for the government, in small law firms, for their own practice, etc. These jobs have more humble salaries that the crazy high ones you read about for those top lawyers. Scientist pay can fluctuate greatly depending on their area of specialty, their credentials (BS or PhD?) and who they work for -- a pharmaceutical company may pay more than a small town Dept of Health.
(d) work/life balance - Law is a notoriously hard career for "having a life" and succeeding, especially when you are first starting out. Science often has its own pressures (to get grants to fund research; to get exciting, meaningful research projects; to publish; to get tenure, if you're teaching). Consider what challenges you are willing to take on to be successful in your chosen field.
Desiree recommends the following next steps: