Another popular website goes iPhone Friendly. This time its the turn of Drugs.com. Here is what it has to say:

Suitable for all physicians and health care professionals, this free, professional version of Drugs.com is designed entirely around the iPhone interface framework. With predictive search, built in drug interactions, and warnings for almost all medications prescribed in the U.S. You’ll need to register with Drugs.com to gain full access. Then, on your iPhone, go to www.drugs.com/iphone.