Medicinal drugs can firstly be classified into prescription drugs, or non-prescription drugs - commonly referred to as over the counter (OTC) drugs. OTC drugs can include remedies for common ailments, or drugs in a lower dosage than would be prescribed by a doctor or nurse practitioner (for example, paracetamol and ibuprofen). These are generally considered to be safe when used correctly and it is not usually necessary for their use to be monitored by healthcare professionals. On the other hand, prescription-only drugs are generally intended to treat more significant conditions, and legally require an appropriately qualified healthcare professional (for example, a doctor or nurse practitioner) to make a diagnostic decision, and then a decision about which drug is most suitable for use in that circumstance. Drugs are generally classified by family or by the physiological effects they can cause. Table 1 shows the two ways that drugs can be described. The effects can either be physiological (i.e. its effects on the body's systems) or therapeutic (i.e. its medicinal use). For example, the drug citalopram could be described by its physiological effect as a selective serotonin-reuptake inhibitor (SSRI), or by its therapeutic action as an antidepressant. There is therefore often overlap between drug classification systems.
| Drug classifications by family (or active ingredient) | Drug classification by physiological effect | Drug classification by therapeutic effect |
| ACE inhibitors | Diuretics | Antihypertensives |
| Barbiturates | Selective serotonin-reuptake inhibitors (SSRIs) | Antidepressants |
| Opioids | CNS depressant | Analgesics |
| Penicillin | Antibiotics | Antimicrobial |
| Heparin (and mimetics) | Anti-coagulants | Antithrombotics |
