Your question is based on multiple misconceptions. First of all, drugs were not widely consumed for recreational purposes. There is very little evidence for what, if any, recreational drug use occurred in Roman society. There certainly was not any kind of "drug trade" comparable to that which exists today, and the Roman state did not have the capacity (or even necessarily the desire) to regulate such a trade if one had existed.

This was a society where the narcotic effects of plants were still not entirely understood, people were legally convicted of using potions and magic, there was literally zero oversight of the medical profession (which was to be sure based on very different evidence and understanding of anatomy and physiology than the modern field of medicine). The Roman state seems to have had a hard enough time regulating matters of state, let alone keeping an eye on the relatively infrequent purchases of expensive soporific ingredients.

Shows like HBO's Rome and Stars' Spartacus give a very misleading, if entertaining, picture of ancient Roman society because of the extent to which they project modern issues onto a distant and very foreign past.

You might find this older answer of mine on drugs in the Roman Empire to be interesting.