Those who have the desire, commitment, willingness to take chances, resilience to endure setbacks, hope for good luck can turn their passion into a career. If you want to be a writer, musician, or whatever just go for it.
However, those who want to remain more grounded and temper their dreams with reality can always make their passion a hobby. Even while studying and practicing medicine you can always write. Take as many writing electives as you can, get a minor in writing or just attend workshops in your area. Get into writing and publishing medical papers, write a medical blog, try to get a medical column in a small newspaper, magazine or site – if you want to combine writing. Manage your time and set aside spare time to write recreationally – there are many channels. If you want a book published, work on manuscripts in spare time and work towards that goal in the sideline.
One of the people I know is a CA by education and profession. However, his passion was always photography. He always took several photos at vacations, events or even everyday stuff. He read books/blogs on photography and would submit photos to amateur contests, publish photos in blogs/FB. Now after a few years of this he has been able to assemble a professional grade portfolio and is considering options for part time/spare time professional work.
The key is to be reasonable and efficiently balance things.