Let's get one item out of the way: Not one of the medical iPhone apps can or should replace seeing a doctor if you are sick or in pain.
That said, WebMD Mobile is an excellent reference tool filled with useful information, including details on drug interactions and first aid tips that could be handy in a pinch. The first aid tool, for example, offers a list of emergency situations and, more critically, information on what to do.
Other benefits: You can navigate through a "symptoms" tab to find an interactive questionnaire to possibly diagnose an illness in your family. Or, using the "treatment" tab, you can search for a drug by name or identify a pill by appearance. Once you find the drug you want, WebMD Mobile details specific precautions and interactions to avoid.
Sadly, WebMD Mobile has a couple problems. You can't search for information within the app itself. Instead, the app launches WebMD's Web site to find the information. This could be a problem if you're on a camping trip in the middle of a forest with no network access.
The second problem is related: You can only launch the app if you're connected to a network. If you're offline, you're out of luck.
WebMD Mobile is the iPhone/iPod Touch companion to the WebMD Web site. WebMD provides medical reference material, health news and community programs to consumers and medical professionals.