Luung, I would suggest you only buy drugs at U-Care (a chain with stores all over the city and one on the Riverside, or Pharmacie De La Gare (see below screenshot). Pharmacie De La Gare is a reputable and popular pharmacy with expats, and it's well stocked. You will pay a bit more at these pharmacies than the other smaller pharmacies all over the place but you are sure to get quality pharmaceuticals. Cambodia had/has a big problem with counterfeit drugs being sold all over the place. There is very little, if any, pharmacy regulation / enforcement in Cambodia so you have to rely on a pharmacy's desire to maintain a good reputation to ensure the quality of medicine.
I bought and used benzodiazepines, antibiotics, and other medicines at these pharmacies and was very satisfied with the quality.
There are very likely other pharmacies in PP where you can find authentic and less expensive drugs, but I was satisfied with these two.