Background and Objectives  Falsified or substandard antimicrobials present a health hazard to patients, and may promote antimicrobials resistance. We conducted a four-year study to evaluate the quality of selected antimicrobials and to examine the prevalence of falsified or substandard antimicrobials in Cambodia, aiming to promote efforts to improve the quality of medicines in Cambodia.Methods   We collected samples of clarithromycin, sulfamethoxazole/trimethoprim, ceftriaxone, cefuroxime, levofloxacin, gentamicin, ciprofloxacin, fluconazole, nalidixic acid, ofloxacin, phenoxymethyl penicillin and roxithromycin products from several different types of drug outlets in five provinces (rural areas) and Phnom Penh (an urban area), during 2011 to 2014. The authenticity of the collected medicines was investigated, and the medicines were analyzed to determine whether they met the appropriate pharmacopoeial standards.Results   We collected 647 samples, produced by 179 manufacturers, from 353 outlets. Only 51 (15%) of the outlets were air-conditioned. We found different-coloured packaging of the same brand (different lots) of products from some manufacturers. The insert information of one sample was different from the package information. Twelve (1.9%) samples were not officially registered with Department of Drug and Food (DDF). In authenticity investigation, 43 of 179 manufacturers replied and confirmed the authenticity of 154 samples (out of 647); also, 18 out of 40 Medicine Regulatory Authority (MRA) replied to enquiries about whether products were licensed or not (one was not). Among the samples, 424 (80.4%), 406 (86%) and 533 (90.6%) passed in dissolution, content uniformity and quantity tests, respectively. Samples of cefuroxime and roxithromycin that failed were significantly cheaper than those that passed.Conclusion  Poor-quality antimicrobials were found in Cambodian markets, though no falsified medicines were detected. Result of samples were not confirmed in authenticity, so it was possible to include falsified medicines. Manufacturers should be encouraged to improve GMP implementation. Storage conditions in the distribution chain may also need to be improved. Continuous efforts by stakeholders are needed to ensure that medicines are properly licensed.