Legally prescribed cannabis is a reality, but road safety rules remain firm....

https://medium.com/@ada_chambers55/can-i-rely-on-the-medical-defence-if-i-was-driving-poorly-but-felt-fine-b45724e80299

Legally prescribed cannabis is a reality, but road safety rules remain firm. Even with a valid medical defence, you must stay under the 2µg/L THC limit and satisfy DVLA fitness-to-drive requirements