You're confused. Cheese can be regulated not because it's practical to block it from being distributed. It isn't, and if you prohibited it you will create a black market.
Only after you've created the black market through prohibition, does regulation then become impossible.
Every single bad thing you can think of about drugs isn't caused by the drugs at all, but by the prohibition. And the few exceptions to this that someone might find... those rare bad things not caused by prohibition itself aren't actually reduced by prohibition. They still happen anyway. The junkie who is ruining his relationship with family still does so today.
People sell bootleg alcohol which may be adulterated with methanol, and it's usually bought on the basis of price. The majority of alcohol is bought from reputable, quality-controlled sources, such as bottle-shops, supermarkets, and bars.
Similarly with drugs: if they're available from reputable, accessible, legal sources, the number of consumers ready to buy from black-market sources will be significantly reduced.
They don't need to block it, people will block it themselves if you give them the information they need. Virtually nobody wants to overdose and die because they thought they were taking oxycodone but got fentanyl instead. These things happen because the user has no way of knowing what they were sold, and because of the ddrug war the only people left selling are the most ruthless.
