Citing the USP-NF is definitely tricky because it is a two-book compilation (the United States Pharmacopeia and the National Formulary) of 4 volumes and AMA doesn’t provide an example for this in the Manual of Style, so we must improvise!
Note: In order to access the USP-NF online, you must create a free Access Point account with your UWaterloo email address. Instructions can be found on this page.
Book Title and edition number. Publisher’s name; Copyright year for the chapter you're referencing. Accessed date. URL to the chapter. (No period after the URL)
Example (using the USP 41-NF 36)