It is outlined in the Drink & Drug Driving (1988) regulations 'Section 5A' titled 'Defences':

Section 5A(3) RTA 1988 provides a defence if a specified controlled drug is prescribed or supplied in accordance with the Misuse of Drugs Act 1971 and taken in accordance with medical advice. Section 5A(4) RTA 1988 confirms that the defence is not available if medical advice about not driving for a certain period of time after taking the drug has not been followed. There is no reverse burden of proof. If a defendant raises this, the Court must assume that the defence is satisfied, unless the prosecution proves beyond reasonable doubt that it is not. The offence in S.4 RTA 1988 applies to those whose driving is impaired by specified controlled drugs taken in such circumstances. Section 4 RTA 1988 also applies to those whose driving is impaired by drugs that are not specified for the purposes of the offence. Given that a defendant may have a medical reason for one drug but not another, it would be sensible to charge each drug offence separately; furthermore, as outlined below, given that the limits are different for each drug, there should be separate charges for each individual drug in any event. Section 5A(6) RTA 1988 provides a defence to being in charge of a motor vehicle with a specified controlled drug in the blood or urine above the specified limit for that drug, if the defendant can show that there was no likelihood of him driving the vehicle while over the specified limit. This is similar to the defence in s. 5(2) RTA 1988.