You can be banned and imprisoned for refusing to do a breath test and subsequently a blood or urine test. I'm not sure what test is done for drugs but it is different to a breath test And legally prescribed drugs are still included with certain levels allowed
I know you can be randomly stopped. It happened to me once years ago tax disk was ok at the scene but pre internet had to take insurance and licence to the police station wasn’t breathalised though. Maybe I might have been if I had smelled of drink It was a pain to make a special trip and almost felt a bit of a felon going in to the station to produce the relevant documents Be even more of a pain these days now they’ve closed most of the police stations. Good that most people stopped were clear
Why does it matter if the drugs are legally obtained? Alcohol is obtained legally - driving under it's influence is an offence