Hello,
I have been a medical writer for 15 years (freelance for the last 10 years) mainly in the area of publications and medical education (some writers also specialize in regulatory documentation and clinical trial reports). I had been a postdoctoral researcher in pharmacology up to that point.
You don't need experience to apply for jobs, but it does mean that you will start nearer the bottom of the ladder (although progression can be relatively quick, especially if you are an MD, PhD or BPharm). Your pay is likely to be considerably less than a GP salary (at least in the beginning). Also, most first jobs would most likely be office based (only more experienced writers tend to have the option to work from home).
There are loads of jobs advertised on the web by medical communications companies, clinical research organizations and recruitment agencies, so just send off your CV (even if there is no job advertised, consider a speculative one, if it is a company you like the look of - that is what I did). You may get sent a writing test to do at home, and if they like the look of it, then they will probably ask you in for interview. Good writers are in high demand, so if you show any aptitude for it, then you stand a very good chance.
If you want to work freelance from home, then you will probably need to have a few of years of experience before anyone will give you any jobs. It's possible that someone might give you a chance straight away, but unlikely, as freelance work tends to evolve from the extended networks you build up when working for a company. Once you get established, though, it can be quite lucrative, and you can charge in the region of £400 per day, if you are any good.
Anyway, let me know if you need any more info (send me a PM, if you like).
Peejoc