Last July when I went to use UpToDate on the iPhone it used the traditional PC website so it was very slow and ill suited to the iPhone. I went to use it a few days ago and it had been reformatted to the iPhone and was fast and clean.
I tap on the UpToDate icon to launch Safari and go straight to UpToDate. The icon is called a webclip. You can learn more about this at this Apple webpage.