Are some prescription drugs illegal?

Are some prescription drugs illegal?

It is an offence to possess drugs such as methadone and benzodiazepines without a doctor’s prescription. However, if a doctor has prescribed Valium for example, possession in such instances is considered legal. However, it should be noted that although it is legal to possess methadone if prescribed by a doctor, it may be an offence to inject the drug due to the fact that methadone is meant to be taken orally in accordance to “recognised therapeutic standards”.