Prescribed cannabis isn't a free pass. A medical defence exists for the 2mcg/L...

https://bizzmarkblog.com/the-motorists-guide-to-medical-cannabis-dvla-rules-section-5a-and-staying-legal/

Prescribed cannabis isn't a free pass. A medical defence exists for the 2mcg/L THC limit, but it’s complex. Your priority is DVLA fitness to drive. Legality isn't a substitute for responsibility; impairment is an offence