It is well established that opium poppies were cultivated in the Roman Empire. It is possible, based on citations in Dioscorides' De Materia Medica, that opium's possible effects generated a legacy of debate, caution, and uncertainty in ancient society.[1]

By the time the Roman Empire was fully established, opium's soporific effects were known and exploited in Rome. Marcus Aurelius, was actually addicted to opium, exact reasons for which are debated but some postulate that it was to overcome stress. [2]

in D. C.A. Hillman's Book, The Chemical Muse : Drug Use and the Roots of Western Civilization, he provides various sources and historical accounts of physicians prescribing opium, sedatives and narcotics etc. His hypothesis at the time was that drugs in ancient Greek and Roman societies constituted an integral element of society and may have favorably developed the course of Western Civilisation.

Personally, i think its a little far-fetched that its being suggested that it was integral to the development of Western Civilisation but that is just my opinion.

TL DR; Opium was relatively common and was not considered taboo, alcohol most likely would have been the other most common one.

[1]https://books.google.co.uk/books/about/Drugs_and_Narcotics_in_History.html?id=I1YxRwBn5poC&redir_esc=y

[2] http://www.jstor.org/stable/2707876?seq=1#page_scan_tab_contents