It's important for any textbook to cite whatever it's made out of, so I'm going to cite appropriately (I hope). Citations will generally be seen at the end of each unit.
A huge source for me are the prescribing information (PI) leaflets that are available for just about every prescription drug in the US. Those are the little leaflets covered in indecipherable jargon (you may have seen them if any of your prescription medications are sold to you in the manufacturer's prepackaged boxes). The leaflets are available for free at RxList.com (not that they're worth the long and difficult read, but in case you wanted to know...).
Another major source I use is eMedicine.com. It's a fairly comprehensive and authoritative website with both professional and consumer information for all sorts of maladies. Each article is written by a healthcare professional (physician, nurse, administrator, etc.) and reviewed by a panel of fellow healthcare workers. I generally trust the site for good information, although the editors do rarely let slip through a few bizarre articles written by the more eccentric healthcare professionals out there.
I occasionally will use personal anecdotes, but don't read these as if they were completely true and/or representative of a typical patient. The anecdotes, really, are more for the humor value.
Please do note that some parts of the site (specifically, the summaries I put up to whet your appetites) generally will not have citations due to their short length.
Also at the end of many units, I'll have available a list of consumer (non-nerd)-friendly resources that are related to the page's subject matter. These will sometimes include consumer information as provided by the respective drugs' manufacturers (though don't expect me to link every single drug out there!).
