Laudanum.

Laudanum created vast swathes of drug addiction throughout Victorian society. It was a tincture made with approximately 10% opium powder.
Laudanum was available over-the-counter from every pharmacy and was dirt cheap.
Whole groups of people were addicted and countless individuals relied on this noxious substance.
Of course - it did have genuine medicinal properties too but overall - it was a social dragdown.
Perhaps we should legalise such substances in today's society, huh?