Is your perspective coming from section 5A (4)? Copied for reference:
(a)contrary to any advice, given by the person by whom the drug was prescribed or supplied, about the amount of time that should elapse between taking the drug and driving a motor vehicle.
At the time of initial prescription advice about driving was sought from the prescribing doctor and from recollection it was indicated that driving was fine as long as not impaired. No limits as to either dosage or time that should expire before taking a dose and driving was indicated.
(b)contrary to any accompanying instructions about that matter (so far as consistent with any such advice) given by the manufacturer or distributor of the drug.
There was no patient information leaflet given with the medication itself. The medications are "specials" i.e. unlicensed and prescribed "off label", so do not have MHRA documentation. The label on the packaging does indicate that one should not drive if feeling sleepy.
Is there a legal requirement for the doctor to attend court and provide evidence?
The clinic published a document about safe driving which was obtained from the pharmacy, but it does not contain limits, only guidance which comes down to the side effects a patient may experience. In particular it states that it is highly likely that if a patient has been prescribed medical cannabis preparations containing THC, the patient will exceed the statutory limit.
Ultimately the patient retains the ability to decide if they are feeling impaired. They take the risk that if their driving is shown to be actually impaired after usage, the medical defence will not apply. The charge would be of driving whilst unfit (the historical section 4 RTA offence).
Usually most side effects are experienced within 1-2 hours of taking a dose. There are no extended release medications yet available which prevent the "ups and downs" associated with vaping prescribed flower and ingesting oil, or in other words, reducing the subjective level of impairment a patient might experience.
In terms of proving whether the amount prescribed was not exceeded, after speaking with a toxicologist this may not be scientifically possible to determine, especially when considering how THC is metabolised. Much would depend on the patient, history of use and amounts, their height and weight and existing tolerance levels. There is a study which shows levels in the blood can be as high as roughly 100 micrograms per litre within roughly 25 - 30 minutes of a dose, which then drops by 90% every hour and a half thereafter. That is why the CPS documentation requires the police to take a blood sample as soon as possible after detaining an individual.
Insofar as needing to "prove" that the amount had not been exceeded, see S5A(5):
(5)If evidence is adduced that is sufficient to raise an issue with respect to the defence in subsection (3), the court must assume that the defence is satisfied unless the prosecution proves beyond reasonable doubt that it is not.
Can you confirm the evidential burden for the defence is therefore 51%, i.e. on the balance of probabilities in order to oblige a court to assume it is satisfied? Of course, this is subject to the reverse burden of proof on the prosecution.
Apologies to ask again, but do you know of any reported cases dealing with this point?