British National Formulary (BNF) chapters and corresponding Read V2 codes
| BNF chapter | Category | Read code |
| 1 | Gastrointestinal system | a…. |
| 2 | Cardiovascular system | b…. |
| 3 | Respiratory system | c…. |
| 4 | Central nervous system | d…. |
| 5 | Infections | e…. |
| 6 | Endocrine system | f…. |
| 7 | Obstetrics, gynaecology and urinary-tract disorders | g…. |
| 8 | Malignant disease and immunosuppression | h…. |
| 9 | Nutrition and blood | i…. |
| 10 | Musculoskeletal and joint diseases | j…. |
| 11 | Eye | k…. |
| 12 | Ear, nose and oropharynx | l…. |
| 13 | Skin | m…. |
| 14 | Immunological products and vaccines | n…. |
| 15 | Anaesthesia | o…. |
The BNF is divided into chapters, each consisting of multiple sections. Read drug codes (version 2) are five digits, with the first letter indicating the corresponding BNF chapter. Medications in chapter 15 (anaesthesia) were not included in this study, as these drugs are not prescribed in primary care.