Usually you cite drug package inserts.
See Chapter 3.13.6 of the AMA Manual of Style (11th ed.):
Package inserts, patient information, and prescribing information (the material about the use and effects of the product) may be cited as follows:
- Zithromax. Prescribing information. Pfizer; 2017. Accessed June 23, 2019. https://www.pfizermedicalinformation.com/en-us/zithromax
- Azilect. Package insert. Teva Pharmaceuticals Inc; 2014.
The general pattern, as I see it, would be:
Drug. Type-of-document. Manufacturer. Date. Optional-internet-access-date. Optional-internet-source..
If there are multiple manufacturers for one drug, you could cite all of them (as long as that seems appropriate for the specific context within your publication).