Prescribed medical cannabis isn't an immunity card. A medical defence exists...

https://blogfreely.net/neasalnfbo/h1-b-the-medication-minefield-how-to-safely-test-new-doses-without-risking

Prescribed medical cannabis isn't an immunity card. A medical defence exists for the 2mcg/L THC limit, but DVLA fitness rules still apply. Legality doesn't replace responsibility; if you’re impaired, you shouldn't be driving. Stay safe and aware.