One of the most popular blogs run by a healthcare professional is Kevin, M.D. - Medical Weblog. Kevin Pho, MD, is a practicing internist and family practitioner. I rely on it for pointers to news of interest, in addition to other insights he provides from time to time.
His blog is especially interesting because he uses it as a way to promote a Web business he runs, KevinMD.com. On this site, he takes questions from e-visitors and provides detailed, well-researched and documented responses. He does not diagnose or prescribe, but the information he provides would be very useful in a visit to your personal physician. For this service he charges $15 on average, more for complex questions.
The blog being such a good read, I'm reminded every time I read it that his service exists. The blog doesn't trumpet the service, but many of us visit About pages when we like a blog, and visiting his pops up a news story about his service. Yet another layer of indirection, but it obviates much of the stigma that "shameless commerce" attaches to a blog.
It's a good model to follow for selling services if your blog is not the primary value you are delivering via the Web. My assumption has always been that most professionals who blog do so for marketing rather than sales reasons - building their personal "brand", so to speak. But if you have a service to sell as Kevin does, or perhaps a line of nutritional supplements or a book or some other health-related product, Kevin's approach looks like it would work pretty well.
His blog gets about 72,000 unique visitors per month, according to the advertising agent for his site, BlogAds. Many of those, like me, are regulars who visit more than once a month.
With that scale of visitation, advertising becomes profitable, and a service like BlogAds is a good way to manage it. Google AdSense is the most common way to put ads on your blog, and TypePad offers to put ads on your blog as a way to pay for your service (from the Weblogs tab, choose Edit Design, then Content Selections, and you'll see Text Ads at the top of the Sidebar Content column).
I'd bet that the number of visitors you need to outweigh the mild stigma ads on your site may create is situation-dependent. Kevin's blog has ads that look tasteful and fit well with his content, and that's another key to reducing the negative effect of ads. In fact, Dmitriy Kruglyak of the Medical Blogging Network maintains that tasteful, well-targeted ads like those on the KevinMD blog actually add value. I'm turning into a convert as time goes by... :-)