Spanish classes can be good, but they take a long time and can be very expensive. If you're going to go that route, I'd recommend at least eight: two introductory (100-level), which should focus on basic vocabulary and syntax; two intermediate (200-level), one focusing on speech and the other on composition; two advanced (300-level), again one focusing on speech and the other on composition; and two specific to the medical field. If your university does not offer such classes, try to find literature courses in which you can choose your own literature, and read medical novels. This should familiarize you with the specific terminology you need.
When selecting a professor, many are tempted to go with one with an Hispanic name. This technique does not always produce good results. For one thing, many native Spanish speakers do not fully understand grammatical principles, just as many English speakers do not, and it may be frustrating to ask why something is like it is and not receive an answer. Second, those who learn Spanish first and then English may not understand as well the errors beginning Spanish students make, especially if they are not as familiar with English idiom. On the other hand, native English speakers may only know Spanish academically, and may not be as familiar with Hispanic culture and idiom as native Spanish speakers are. My professor was a native speaker of both languages, and grew up in Puerto Rico. He seemed to have a good balance of both worlds.
I've heard good things about Rosetta Stone, but I've never used it myself. I'm not entirely convinced of the ability of a computer (a mathematically-based instrument) to be able to teach language (a decidedly unmathematical field). I think you'd be far better off actually speaking with a live person. It's for this reason I strongly recommend against online Spanish courses. You need the interaction with a professor and a class to practice your skills.
Just a question: Are you planning to go abroad with Medicos sin Fronteras?