Prescribed medical cannabis doesn’t grant immunity. You still face the 2mcg/L...

https://wiki-global.win/index.php/Driving_an_EV_on_Unfamiliar_Roads_While_on_Medication:_A_Guide_to_Staying_Legal_and_Safe

Prescribed medical cannabis doesn’t grant immunity. You still face the 2mcg/L THC limit, and while a medical defence exists, DVLA fitness to drive rules take priority. Legality isn't a blanket pass; impairment remains your responsibility