Are you trying to write freelance medical articles on the side, or looking for someone to write for you?
Check out Elance.com....A lot of times, I see postings for this, so if you have work samples, then put in a bid for the project. Hope this helps!
You won't get paid a lot, but you can build credibility in your writing by joining a network like About.com - and I know in their Health channel they do try to hire doctors.
http://beaguide.about.com/ I think Latoya here is currently a Guide with them, so she might be able to tell you more about what they're like to work with at the moment.
I refrained myself from joining About.com a while back because I feel I need too much to learn before aspiring become a guide. However I'm considering joining this site upon approval www.suite101.com/freelance_writers
I wrote for About.com - my first Web writing client - despise the company, but I'll admit they're an OK place to build your reputation in a niche for a little while. I was an Editor for Suite101. I'd suggest avoiding them like the plague. If you visit my blog at www.allfreelancewriting.com, I'm sure you'll be able to find a post or two explaining more about why I'm saying that.
If I have to be honest, Academia Research came across to me via an Adsense link, but once on-site I found them too pretentious to try. The OP asked for a medical-related gig and such is the only I have seen so far, though Jenn's suggestion reminded me About's multiple channels. Anyway, your feedback on Academia Research makes us aware to avoid them too, thank you