INTRODUCTION

Two systems are commonly used in drug utilization studies to classify/group drugs, or Drug Identification Numbers . Both systems require updating on a regular basis.


1. Anatomical Therapeutic Chemical (ATC) Classification System
   
   
2. American Hospital Formulary System (AHFS) Pharmacologic-Therapeutic Classification System®

The AHFS system of classification is more commonly used in North America while the ATC system of classification is more commonly used in European countries. The primary differences between the AHFS and the ATC systems are that the ATC system is: a) able to more finely group drugs (or their DINs) and b) includes information on the defined daily dose (DDD) .

The AHFS system uses a three-level step-up / step-down method for grouping drugs while the ATC system uses a five-level step-up/step-down method. The DDD measure can be used to determine intensity of use at a varying level of detail - for example, for nervous systems drugs, the ATC classification provides the following information:


• 1st-level: nervous system drugs (the broadest level)
• 2nd-level: psycholeptics
• 3rd-level: anxiolytics
• 4th-level: benzodiazepines
• 5th-level: diazepam (the finest level of information)