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

https://wiki-spirit.win/index.php/Is_It_Risky_to_Drive_Right_After_Taking_My_Dose_Even_If_I_Feel_Normal%3F

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.