🧠The theory: risk is harm times likelihood
A medicine is high-risk when a small error produces a large, sometimes irreversible, harm. Three features drive this: a narrow therapeutic index (the effective and toxic doses are close), the seriousness of the harm when it goes wrong (major bleeding, respiratory arrest, marrow failure), and how error-prone the medicine is in practice (unusual dosing, look-alike names, complex monitoring). Systems such as monitoring, brand consistency, alerts and counselling exist to reduce the likelihood, because the harm cannot be reduced.
Common features of high-risk medicines
- •A narrow therapeutic index, so the dose that works and the dose that harms are close together (for example digoxin, lithium, phenytoin, aminoglycosides).
- •The potential for serious, sometimes fatal, harm (for example anticoagulants, insulin, opioids, methotrexate, chemotherapy).
- •A need for regular monitoring of blood levels, organ function or clinical response.
- •Unusual or error-prone dosing, such as weekly methotrexate or units of insulin.
- •Many clinically important interactions.
Groups you must know
| Group | Examples | Main hazard |
|---|---|---|
| Anticoagulants | Warfarin, DOACs, heparins | Bleeding |
| Insulin | All insulins | Hypoglycaemia; dosing errors |
| Opioids | Morphine, oxycodone, fentanyl | Respiratory depression |
| Methotrexate | Oral methotrexate | Marrow suppression; weekly dosing error |
| Lithium | Lithium carbonate/citrate | Toxicity; narrow index |
| Antiepileptics | Valproate, phenytoin, carbamazepine | Teratogenicity; narrow index; interactions |
💡 How to use this guide
Each chapter follows the same pattern: why the medicine is high-risk, the counselling points, what to monitor, the key interactions, and the signs of toxicity. If you can recall those five things for each drug, you can answer most exam questions on it.
Test yourself
Give three features that tend to make a medicine high-risk.
Answer: A narrow therapeutic index, the potential for serious harm, a need for regular monitoring, error-prone dosing, or many important interactions (any three).
Why is weekly methotrexate considered high-risk even though the dose is small?
Answer: Because the unusual weekly schedule is easily mistaken for daily dosing, and daily methotrexate can cause fatal bone marrow suppression. The error is likely and the harm is severe.